Managing High Pond pH (How to Lower & Stabilize pH)

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Koi in pond
Pond fish, such as koi, are particularly sensitive to changes in water pH. Nikita / CC BY 2.0

“What’s my pond’s pH?” is a question every pond owner should ask regularly. An abnormal pH can significantly damage a pond’s ecosystem. Defined literally as the potential (or power) of hydrogen, pH is a measurement of how acidic or basic a liquid is. The pH of most liquids falls between 0 and 14, with a pH of 7 being considered perfectly neutral. A liquid with a pH below 7 is acidic, and anything above 7 is basic, or “alkaline.” Pure water has a pH of 7.

No pond will ever be perfectly neutral. A pH anywhere in the range of 6.5 – 9.5 is typically fine, though a pH of 6.8 – 8.5 might be best for koi ponds. Most ponds fall within this range naturally, but in some instances, pH can rise or fall to abnormal levels (usually as a result of human activity). Unusually acidic or basic conditions can severely impair fish growth and reproduction, and a pH below 4 or above 11 is typically fatal. Even within levels that fish normally tolerate, sudden, extreme shifts in pH can also be damaging. Keeping pH levels relatively consistent is just as important as keeping them close to neutral.

Mathematically, pH equals the negative logarithm of the concentration of hydrogen ions, or -log[H+]. In everyday terms, this means that every number on the pH scale represents a 10-fold change in acidity and alkalinity. The more positively charged hydrogen ions (H+) in the water, the more acidic a pond is, and the smaller the pH will be. Conversely, both basicness and pH values increase as the concentration of H+ ions decreases. Therefore, water with a pH of 8 is 10 times more basic than water with a pH of 7, while black coffee (pH of 5) would be 100 times more acidic. H+ ions are highly reactive; in a pond environment, they quickly form bonds with other substances. This property is responsible for many of the problems aquatic organisms face from very acidic conditions.

Just as problematic, though, is pondwater that becomes too basic. High pH levels can be particularly problematic because they can increase the concentration of toxic ammonia, NH3. Both ammonia and its nontoxic derivative, ammonium (NH4+) are natural waste products of fish. The bulk of NH3 and NH4+ is generally removed through normal filtration by beneficial pond bacteria, leaving NH3 concentrations too low to affect aquatic organisms. Alkaline conditions, though, can cause NH3 concentrations to spike to unnatural levels. That’s because the low concentrations of H+ ions in high pH waters increase the likelihood that nontoxic NH4+ ions will lose a hydrogen ion, resulting in more toxic NH3. If the pH is high enough, the subsequent NH3 concentrations can make fish sick and lethargic, possibly stunting their growth or even killing them.

The only way to know your pond’s pH is to physically test the water using a testing kit or an electronic reader. Testing pH regularly is a good habit that can help you detect any problems early. How often you test is up to you, but anywhere between weekly and monthly is probably fine. Of course, if you notice any sudden changes in water quality or algae growth, or if your fish are acting strangely or die unexpectedly, it’s a good idea to test your pH as soon as possible. If you can, try to always test your pH at the same time of day. For reasons we’ll explain later, pH naturally tends to rise during the day and fall at night (though pH shouldn’t fluctuate by more than 0.8 per day). Checking at a consistent time will help to keep you from mistaking normal fluctuations for real issues.

What Causes High pH in Ponds?

Algal bloom in pond
Algae blooms are one of several possible reasons why your pond pH is unusually high. Alabama Extension / No copyright

To fully understand the causes of high pond pH, a background in the basics of the underlying chemistry is extremely helpful. We mentioned previously that pH falls when the concentration of hydrogen ions, or [H+], increases, and that it rises when [H+] decreases. In waters exposed to the elements, [H+] is affected by a variety of chemical interactions. One of the primary drivers of [H+] changes is carbon dioxide, or CO2. CO2 can enter your pond both from contact between water and the atmosphere and as a byproduct of plant and animal respiration. Upon entering the pond, CO2 mixes with water to form H+ ions and bicarbonate, or HCO3:

CO2 + H2O = H+ + HCO3

As shown by the equation above, the introduction of more CO2 results in more H+ ions and therefore a lower pH.

On the other hand, the removal of CO2 can cause the pH to rise. When free CO2 is removed, the equation shifts to the left — that is, H+ ions and HCO3 are more likely to recombine, forming more CO2 (and H2O) to restore the balance. That reduction in [H+] results in a higher pH. Photosynthesis by plants and algae can remove significant amounts of CO2, causing pH to increase. Because photosynthesis requires sunlight, pH tends to rise during the day and peak around twilight. At night, when respiration outpaces photosynthesis, pH steadily falls again, reaching its lowest point around dawn.

Another factor affecting pond pH is alkalinity. A pond’s alkalinity is its ability to buffer, or reduce, [H+] concentrations. Buffering happens when negatively charged ions bond with H+ ions and thereby neutralize them. In ponds, the most common buffering ions are HCO3 and carbonate, or CO32-. As these molecules bond with H+ ions, [H+] decreases, causing pH to rise. The specific concentration of HCO3 and CO32- in a pond (measured in parts per million, or ppm) is called carbonate hardness, or KH. Like with pH, you can find out your pond’s KH using a testing kit. KH itself doesn’t directly impact fish, but high carbonate hardness can result in a high pH that’s difficult to lower. On the other hand, a KH that’s too low will make your pond vulnerable to sudden pH shifts. For fish ponds, a desirable KH range is 125 – 180 ppm.

Ordinarily, all factors more or less balance out, and pH stays within a safe range even as it fluctuates somewhat in a 24-hour period. However, sometimes, an issue arises that throws off the natural balance and pushes pH too far upward. Often, high pH problems are a result of sudden spikes in plant and algae growth, or “blooms.” For example, fertilizer that is intentionally added to a pond or that enters a pond through runoff can cause plants and algae to grow very quickly, photosynthesizing and raising pH as they do. When this happens, pH can reach unsafe levels during the day and may not decrease as much at night. In general, any human activity that causes an inordinate rise in photosynthesis is also likely to result in highly basic water conditions.

Very commonly, pH can also rise when pond water is in direct contact with untreated cement. Over time, untreated and/or uncovered cement will leach calcium carbonate (CaCO3) out into the water. When CaCO3 dissolves, one of the results is an inordinate amount of free CO32-, which can cause pH to increase. The same effect can happen when pond water is exposed to brick, mortar, or any other surfaces or ornaments made with materials that contain carbonate. KH levels that rise on pace with pH may be a sign that this issue is occurring.

Testing Water for pH and KH

Water testing kit
Commercial water test kits can give you a general idea of where your pond sits in the pH range, while an electronic pH tester can give you more accurate results. Jellaluna / CC BY 2.0

The only way to determine if you have a high or low water pH is to test! Common commercial water test kits are generally color-coded and only provide a wide-range pH measurement. This is generally within a scale of 4.0 to 9.0, with increments of 0.5 for each color. These tests are fine for most purposes, and they can at least give you an idea of where your pond sits in the pH range.

If you want more accurate pH measurements, to a resolution of 0.1 or 0.01, you’ll need to invest in an electronic pH tester. These only test solutions for pH, but they have the advantage of being able to provide measurement values up to 2 decimal points.

Either choice would be fine in most cases, so you can just choose the option that is the most convenient or cost-effective. Alongside pH testing, we also recommend testing for water KH, as this acts as a buffer that stabilizes (or destabilizes) pH in water depending on the range. A change in pH can be directly related to KH, and to re-stabilize pH, you may need to also adjust water KH.

For more information on water testing and our recommended test kit choices, see our full guide here.

How to Lower Pond Water pH & Prevent Rapid pH Spikes

When it comes to pH problems, prevention is always more effective than trying to fix the issue after the fact. There are several ways to lower pH, but it’s a good idea to address any potential causes of high pH as soon as possible. If you have a cement pond, make sure the cement is properly treated before installation to prevent substances from leaching out into the water.

Additionally, you can avoid adding fertilizer whenever possible to prevent plant and algae blooms, which will prevent CO2 from becoming depleted due to photosynthesis. If you absolutely must use fertilizer, try to do so at least several weeks before adding fish. Doing so will ideally provide enough time for the initial, largest algal bloom to pass and allow your pond’s ecosystem to stabilize.

1) Stabilize Water KH

Gypsum powder
Adding gypsum to a pond with high KH levels may not impact KH directly, but it can improve water hardness, which may help to prevent high pH in the future. Brian Shiro, CC BY-SA 3.0, via Wikimedia Commons

If you suspect that your pond’s pH is rising, a KH greater than 180 ppm could be the culprit. The ideal solution to this problem would be to identify and remove the source of the excess carbonates, such as removing or covering the offending surface/structure/ornament. Of course, in some situations, this is easier said than done. For instance, if your KH is high because your pond is made with untreated cement, thoroughly eliminating the problem might involve draining your pond and/or investing in some expensive improvements.

For a short-term fix, you have several options. If you have the time and energy, and if your pond isn’t too large, you can reduce KH by habitually performing partial water changes. Assuming that the water you add is of an appropriate KH, this should reduce carbonate concentrations, although you’d have to do it regularly for as long as the underlying issue persists. An easier choice may be to add something to the pond that will lower both KH and pH. Adding CO2 or controlled amounts of an acid can help to improve KH and reduce pH, although neither will address the underlying problem (we’ll talk more about these later).

If your KH is too high, you can also try adding gypsum (also known as calcium sulfate, or CaSO4). Gypsum isn’t likely to impact KH directly. Instead, it should improve water hardness. Here, “hardness” refers to the levels of certain minerals like calcium and magnesium dissolved in water. Some evidence suggests that ponds with high KH and low hardness are more likely to have high pH issues. Whether raising water hardness, through adding gypsum or another method, will actually lower an already high pH is arguable. However, it may help to prevent high pH problems in the future, and at the very least adding extra calcium can be beneficial for aquatic life. If you’re interested in trying gypsum, you can calculate the amount you’ll need using the following formula:

[Alkalinity (in mg/L) – Hardness (in mg/L)] * 2

So, if your harness is 20 mg/L and your alkalinity is 80 mg/L, the amount of gypsum you’d need would be:

[80 mg/L – 20 mg/L] * 2 = 120 mg/L

Multiplying this answer by the volume of your pond would give you the mass of gypsum you’d need. Large amounts of gypsum can get expensive, so this option may only be economical for smaller ponds.

  • Safe for aquatic life
  • Can raise low hardness levels
  • Provides beneficial extra calcium for all pond animals
  • May help to prevent future high pH problems
  • May not be effective for lowering an already high pH
  • Expensive in large quantities
  • Won’t impact KH directly

2) Remove Algae and Excess Weeds

Green pond water
Very green pond water may be a sign that your high pH levels are being caused by an algae bloom. Lisa Ann Yount / No copyright

If your pond’s water is very green, your high pH is probably the result of an algae bloom. In this case, the very first thing you should do is halt or limit fertilizer use and identify and address any areas where runoff could be entering your pond. If you’ve done this and the pH is still too high, the next step is to try to remove as much of the algae and excess weeds as you can. If this is done, your pond should gradually return to a healthy pH.

For most types of ponds, especially those with fish, we always recommend a UV clarifier for algae removal and control. UV clarifiers, unlike algaecides, are very safe for fish and are also very effective at controlling free-swimming green water algae. Running a clarifier for a few days should clear almost all green algae, and then the water can be re-tested to see if pH has stabilized and lowered. Sometimes the removal of algae is enough to bring pH down and stop rapid changes occurring between the morning and night.

Removing excess weeds can also slow the overabundant photosynthesis that may be contributing to high pH. Herbicides are inadvisable, as they can harm fish and kill more plants than you intend. Safer options include manually removing weeds, and temporarily blocking sunlight partially. The former method can be time-consuming and labor-intensive, while the latter typically requires specially designed dyes, or aerators that stir up sediment to obstruct light. These methods are less effective than herbicides, especially in larger ponds, but they pose less of a threat to your aquatic ecosystem.

Algae/Weed Removal
  • Long-term fix for the most common underlying cause of high pH
  • Can reduce drastic day/night pH differences
  • Manual methods and UV filters are safe for aquatic life
  • Once installed, UV clarifiers require no manual effort yet work continually
  • Promotes a healthier ecosystem for all pond life
  • Makes pond look more aesthetically pleasing
  • Algae and weeds will return if fertilizers continue to enter pond
  • Manual methods may be time-consuming and labor-intensive
  • May require special dyes or aerators to reduce sunlight penetration
  • Herbicides and algaecides may be very dangerous for fish and other animals, and herbicides may harm desirable plants

3) Increase Acidity of Water

Algae in water
Adding alum to your pond can cause algae to clump together around it and precipitate out of the water. NOAA Great Lakes Environmental Research Laboratory, CC BY-SA 2.0, via Wikimedia Commons

Once your water KH is in the range of 125 – 180 ppm and algae and weeds are under control, you can consider lowering your pH using a safe acid compound. Before doing this, we recommend testing your pond water pH in both the morning and night for a few days. If your pH readings are within 6.8 – 9 and there are no major swings each day (more than 0.8), you likely do not need to lower the pH further. A stable pH is more important than a changing pH, and as long as the pH is within the safe range it should be fine for pond fish.

When you still need to lower your pH but can’t do so by correcting the underlying problem, you can turn to an acid. If your KH is also high, you may need to add a significant amount of acid due to your pond’s current buffering capacity. One of the best options is alum, or aluminum sulfate (Al2(SO4)3). Alum is a fairly safe acid, and although its effects on pH are only temporary, it can help make your pond less alkaline until you get the true issue under control.

Alum can also help your pond in other ways. Primarily, it causes algae to clump together around it and precipitate out of the water; from there, these clumps of algae will sink and become part of the sediment. Additionally, alum can remove excess nitrogen and phosphorus, which are often responsible for the algal blooms that raise pH in the first place. Multiple treatments with alum may be necessary to reduce pH for any meaningful amount of time, but the compound is relatively inexpensive.

If you decide to try alum, we recommend starting with smaller amounts, as too much can cause a dangerous pH crash (though the same is true for any other acid). You might start with 10 mg/L and wait to see if that helps. If pH needs lowering again later, try adding more alum in doses of 5 – 10 mg/L at a time. Avoid adding alum, or any other acid, altogether if your KH is very low, since this would make your pond vulnerable to a violent, unsafe pH drop.

Alum (Aluminum Sulfate)
  • Fairly effective for reducing pH
  • Safe for aquatic life
  • Helps to remove algae
  • May remove excess nitrogen and phosphorus
  • Relatively inexpensive
  • Only a temporary solution
  • Multiple alum treatments may be necessary
  • Needs to be added slowly to prevent pH crash

Another option we suggest is a “pH down” product. These products contain bio-safe compounds which slowly increase the acidity of the pond water. The benefit of using these products is that they have already been tested for fish safety, and they generally only adjust pH by very slow amounts. Gradually lowering your pH is better than a rapid decline, and most of these treatments only lower by 0.2 – 0.5 in 24 hours.

We recommend API’s pH Down formula for safe pH lowering in ponds. This product contains low-dose sulfuric acid which will help neutralize alkaline substances and lower the pH of water over time. After dosing and waiting 24 hours, it is advised to re-test pond water in the morning and night to see the new pH values. If the pH needs to be lowered further, a repeat treatment can be applied to the pond.

Low-Dose Sulfuric Acid (“pH Down” Products)
  • Fairly effective for reducing pH
  • Safe for aquatic life
  • Lowers pH very gradually (0.2 – 0.5 in 24 hours)
  • Relatively inexpensive
  • Only a temporary solution
  • Repeat treatments may be necessary
  • May not be economical for very large ponds

4) Add CO2

Soybean meal
Adding an organic substance, such as soybean meal (pictured), to your water can help to increase CO2 levels. United Soybean Board / CC BY 2.0

The final strategy we recommend is adding CO2 to your pond. As we explained previously, CO2 interacts with water to produce H+ ions. Therefore, adding more of this gas will lower a high pH. To get more CO2 in your pond, all you have to do is add some kind of organic substance, such as cracked corn, soybean meal, or cottonseed meal, to your water. All organic matter contains carbon, and as it decomposes, much of that carbon is released in the form of CO2.

Adding approximately 15 lbs/acre per day for about a week should produce the desired results. This method is dependable, very safe, and likely to reduce pH slowly and steadily rather than drastically. Our only word of caution is that it is possible to add too much organic matter per day.

In addition to producing CO2, organic matter also removes dissolved oxygen from the water as it decays. Excessive amounts of decomposing organic matter can dangerously deplete dissolved oxygen, which might be just as bad for your pond’s ecosystem as a high pH. As long as you regularly measure your pond’s dissolved oxygen concentrations and avoid adding more than 50 lbs of organic matter per acre per day, though, you should be absolutely fine. If needed, aerating your water can also help keep dissolved oxygen levels up.

CO2 (via Organic Matter)
  • Effective and reliable for reducing pH
  • Can be a longer-term solution
  • Should take no longer than 1 week
  • Safe for aquatic life
  • Lowers pH slowly and steadily
  • Relatively inexpensive
  • Easy to add to pond water
  • Excessive amounts may deplete dissolved oxygen (limit 50 lbs/acre/day)
  • Aerating pond water may become necessary
Chris G
About the author

Chris G

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners. Experienced with pond installation, fish stocking, water quality testing, algae control and the troubleshooting of day-to-day pond related problems.

Read more about Pond Informer.

46 thoughts on “Managing High Pond pH (How to Lower & Stabilize pH)”

    • Hi Peter,

      Muriatic acid (hydrochloric acid) is an extremely strong acid and I would personally not recommend using as it has a high chance to produce massive pH crashes – which would be deadly to fish. The only time I could see it being helpful would be in much larger bodies of freshwater, such as for use in lakes. In ponds it would simply be far too difficult and risky to dose safely, especially if you have fish.

      • Hi
        Would it not work if you slowly add the acid over a series of days? I know that the rapid change in PH kills the koi.
        What I am attempting is, to prepare the water first with a conditioner and acid to adjust ph. Letting it stand over night. Then going to do a 30% water change on my pond. My pond is only 250lt. With 4 4 koi. But Boer hole water here is high in metals. Sorted that issue but now causes a high ph of 8. OMG

    • Hi Abbey,

      Citric acid is a fairly weak acid, and so if you also have a high KH/alkalinity buffer (common with high pH) it may not be enough to lower that pH reading. Out of curiosity, have you performed a KH/Carbonate-hardness test? What are your other water quality readings like, such as ammonia and nitrates?

    • Deb, I want to put koi in my 1600 gallon pond. I read one place where it says to use the vinegar like you did and then in another place it says it is NOT safe for koi. Did you use with koi and any fish in your pond?

      • Hi Linda,

        White vinegar and/or distilled vinegars should be safe for ponds, but they’re actually very weak acids, which means they may not be able to lower pH at all if you have high water alkalinity (the acid would simply be buffered).

        Out of curiosity, what kind of pH readings are you seeing on your water quality tests? Also, do you know your water alkalinity/KH parameter?

    • Great idea worked a treat a very distinct drop over night added 1 1/4 cups in total over two dats now have a PH of 7 thanks for the tip KELV

  1. I am trying to get a pool ready for koi. It is a container, preformed 50gal. I have Japanese snails as well. I have a full test kit. I’m showing my ph headed toward 8.4, but the ammonia, nitrite, and phosphates are all at either zero or barely above zero. Does this sound possible? I thought one would affect the other… Should I do the sodium bicarbonate? The water is very clear, and I have a new waterfall filter. The filter media says it is charcoal and calcium balls layered with a coarse filter. Advice or ideas?

    • Hi Jenn,

      Just to double-check, did you mean to say 500 gallons, not 50 gallons? I’m afraid if it’s the latter, that size of pond is far too small, even for a single koi (arguably even most goldfish). Having a koi in such a small pond will cause all sorts of problems, and the fish is unlikely to survive very long. I’d consider looking into much smaller species, such as minnows, mosquito fish, or maybe even 1-2 of the smaller goldfish species. More recommendations here –

  2. Hello:
    I have an in ground pool which is now a koi pond.
    It’s 19′ by 40 ‘ and average depth is 5 feet.
    It’s been in operation about 12 years.
    This year they no longer show them selves nor do they come up to eat.the photo is 9.0+ all other tests are normal.
    There is a lot of algae !
    I bought a lot of various chemicals on Amazon no help !
    I’ve had a air bubble machine for several years.
    Any suggestions on how power the photo and how to fix the algae issue ?

    • Hi Barry,

      I’m sorry to hear about the issues you’re having!

      If you have them, could you let me know all the most recent water quality test results? Also, when you say “photo”, do you mean phosphorus or pH? Finally, what kind of algae are you seeing – free-swimming algae or filamentous algae?

  3. Love your blog and recommendations. I have pool experience 30 yrs owning and operating a 25,000 gal swimming pool. I have recently moved and now have a 500 gal water feature with waterfall, no filter or skimmer that I want to make a water garden with fish. Its about 18 years old best I can tell and never has had plants or fish. I am in start up trying to get the water stabilized. I have added some plants and they are doing ok. Bought your recommended test kits and started testing the water. The pond has a water addition float valve so level is constant. Feed water analyzes at pH 7 Alkalinity 180 hardness 250. My problem is pH spikes I am using muriatic acid adding small amts but can not keep the pH down. Yesterday I added about 1/2 cup in Am and measured the water in the pm, pH 8 AK 80 H 250. Measured again this am and pH 9 Ak 80+ H 250+ ( waiting for my Ak H test kit and am using strips for Ak H). No measurable NH3, NO2 or PO4 in pond.

    Water has fair amt of swimming algae and water is greenish but I can see the bottom and there is no real build up of algae on waterfall or rocks or muck on bottom. I don’t think the pond has a liner so fair amt of exposed concrete.

    Have purchased a Pond Guy G2 1250 filter and UV to help with clarity and started adding beneficial Bacteria

    Do you have any advice on stabilizing pH. I don’t want to introduce more plants or fish until I can control the pH. Only way I can keep it down now is to add strong acid 1/4 to 1/2 cup ~ three times a week. Tried vinegar not strong enough.

    • Hi John,

      I’m sorry to hear you’re having issues with your new pond!

      Can I ask, is your water source mains or well water? Did you happen to record the parameters (GH/KH/pH) of the pond on the very first water quality test, before you began dosing the acid? Does the pH drop even slightly over night, or does it hold out (or continue to rise) until you dose more acid?

  4. Trying to get baseline data after getting both recommended test kits. Feed water measures pH 7, KH 200, GH 300. The measurements below are pond water. pH definitely increases with time after acid additions and from AM to PM. I think if I just left it alone it would stay pretty much at pH 9. Any advice/opinions appreciated. I would like to stabilize at pH ~7.5 without adding so much acid since if I am away for a few days now pH will certainly go to 9. Adding over a cup of muriatic a week at present to ever see a pH below 8.

    Date pH NH3 NO2PO4 KH GH AM/PM Acid additions
    7/1/2019 8.0 0 0 0 80/strip 250/strip PM Added 1/2 cup
    7/2/2019 9.0 80+/strip 250+/strip AM
    7/4/2019 7.5
    7/5/2019 8.0 0 0 0 120 360 AM Added 1/4 cup
    7/5/2019 9.0 PM
    7/6/2019 8.0 180 220 AM
    7/6/2019 9.0 100 380 PM Added 1/2 cup
    7/7/2019 7.5 80 340 AM

  5. Maybe this reads better
    pH KH GH AM/PM
    7/1 8.0 80 250 PM
    7/2 9.0 80 250 AM
    7/4 7.5
    7/5 8.0 120 360 AM
    7/5 9.0 PM
    7/6 8.0 180 220 AM
    7/6 9.0 100 380 PM
    7/7 7.5 80 380 AM

  6. Probable answer: No easy solution. 500 gal pond with feed water at pH 7 GH 300 KH 200 ( Arizona water hard and alkaline). Pool loses 100 to 200 gal per week to evaporation that is automatically replaced. If left alone, it will tend toward feed water specs except with no liner it will have higher GH and corresponding higher pH. So may have to live wit that. So…What plants and fish live at pH 9 and 380 GH. I can confirm that algae does not sure about much else. Although my lemon drop (toothache plant) is doing well as is the four leaf clover floating water cover. Frogbit not liking it.

    • Hi John,

      Thanks for taking the time to post all your parameter data. That constantly spiking pH is certainly concerning. The combination of your high alkalinity feed water and the exposed concrete are both likely contributing to different degrees, especially if the concrete has become cracked or damaged in places allowing water to permeate.

      You’re right, I don’t think there is an easy solution here, but have you considered draining the pond and fitting a pond liner over the concrete? Although your feed water parameters are high in alkalinity and hardness, this alone shouldn’t be a problem for most fish (arguably, 100-200 ppm KH/alkalinity could be considered an ideal range). However, that large pH change when the water hits the pond is more a problem, but also something you may be able to solve, or at least lessen, with the addition of a pond liner.

  7. I have considered a liner but the problem is how the pond was constructed. Most of the pond bottom has stones/river rocks cemented into it and would be tough secure a liner over. The deep section is just concrete. I have considered draining and using something like roofing tar to coat the bare concrete. Concrete does not show signs of cracking.

    Feed water is from local water company not a well. It is very consistent in pH GH KH. Most homes have softener systems like mine, Salt exchange, that reduce the hardness of tap water from 300+ to 50 ppm or less. GH test solution never turns pink it’s blue from the start so very low GH.

    When I first arrived and tested the pond water I was using pool test strips and the pH was off scale 8+ and the GH/AH were high and water was green. From my years of swimming pool experience I immediately added about two cups muriatic and several tablespoons of powdered shock (hypochlorite) resulting in nice clear water with pH ~ 6.5-7 as expected with high but in range GH KH.

    Decided to make a water garden and that’s when the issues surfaced. For now I think I will stop adding acid and see where the water quality stabilizes and what happens to my plants. pH does swing daily from ~7.8-8 in the cool mornings to 9 in the hot afternoon if I don’t add acid. I have added the G2 1250 filter/UV and in a day the water is clear with no greenish tint so maybe I can control free alage and turbidity. I’ll add an aerator and try some goldfish if most of the plants survive, I will post again in a week or two with more results. Maybe by then I will know what plants do well and if goldfish can adapt to this unfriendly but stable and well aerated, filtered and algae controlled environment. May prove to be a good experiment for others with this water problem. I am keeping a spreadsheet with all observations Thanks for your time to consider my issues.

    • It’s likely to be the concrete (cement) leaking Lime into the water (in my opinion), or other run-off from surrounding land(s). Drain. Over cover with more cement to level off. Apply curing chemicals to seal the cement & cover with a pond liner.

  8. Minor but very encouraging update. Now that the G2 1250 filter/UV has done it’s work the water is crystal clear and this allowed me to spot ~20+ smallish snails, 1/4-1/2″, working away on cleaning my rocks. The encouraging part is they are living and reproducing at high pH and GH. This gives me hope that a few fish might make it too. Also can see clearly that some of the plants are sending out long rhizomes looking to propagate as well. Overall encouraging observations.

  9. Actually this is the most thought out article on high pH I have seen. And a lot of the cause can be attributed to daily photosynthesis levels removing CO2. This is what I have seen pH 8 in the AM and pH 9 by late afternoon. And yes adjusting with acid is futile and labor intensive.

    Aluminum sulfate, alum, addition is also new to me.

  10. Ok, about 14 days with no acid addition and AM pH has been very stable at 8 with PM rise to 9. Water is very clear with G2 filter/UV. Aeration has been added. GH has trended down from 380 to 320 and KH is trending up from 110 to 160. Of seven plants Golden Club and Frogbite are doing poorly while Lemon drop and floating four leaf clover are doing really well and spider lily, hornwort and horsetail rush are stable to improving. Added 5 ~3″ Comets one died but he was not well right out of the bag, kept sinking, and was gone in two days. The fish hide a lot and spend all their time around the hornwort at the bottom. There are also frogs and snails that seem to be doing fine. I did not add them.

    Adding bacteria but no sign of a nitrogen cycle with NH3, NO2 always reading non detect as well as PO4. I am making no water change outs. In the Arizona heat and wind I estimate I am adding 100+ gallons of water a week to maintain level lost to evaporation.

    Please give your general opinion and advise on how I should proceed. How do I get the nitrogen cycle started? Should I keep adding fish? Would you add any more plants this year or wait until next year. I can’t get any more shipped until September due to heat. Is adding bacteria useless with no ammonia.

  11. About a month has passed and water is relatively stable at pH 8, GH 240, and KH 170 with no acid additions. Still see AM to PM pH increase so water stays above pH 8 a fair amount of the time. Had one small NH3 spike followed a day or so later by an NO2 spike but other than that they have been 0. Bacteria must be working.

    I have 8 fish 6 Comets, 1 Fantail and one Plecostomus. The Pleco has doubled in size in about 2 months and won’t be able to hide that much anymore if he keeps growing! The other fish are growing too. I have added fish twice and see some mortality in the first one or two days but the survivors are thriving with no signs of fin rot or other issues. The comets are pretty stressed when I buy them with 50+ in a small (less than 10 gal) aquarium at the store so a couple dying right off is probably to be expected.

    With fertilizer the plants are doing better. The Mystery Snails I added are growing. Two in particular have grown quite large. Have seen other “bugs” in the pond, Water Boatman and dragonfly larvae plus the indigenous tadpole snails are doing well.

    So far the higher than ideal pH GH KH levels appear not be an issue with good filtration/UV, 2 aeration stones plus waterfall and fountain, and beneficial bacteria addition to an unlined 500+ gallon pond. Water is crystal clear and the fish and snails seem to be keepin the string algae under control.

    Will report back in a month or two but seems that I can have a nice pond with the poorer water quality I have to work with.

  12. Last comment. Also two months has passed since last post with many water measurements. Average AM pH – 8.2 with spike of about 0.6 in PM daily. Average daily pH is about 8.5. Average KH 160 Average GH 280. One small NH3 spike otherwise NH3 and NO2 are zero. All plants doing well except a Golden Club that has leaves on it but not growing to standard size. Predator attack got one GoldFish and my three frogs. The 7 other fish have at least doubled in size with at most daily feeding ( left them for 10 days on their on). They are very active come to me to be fed. Most of the Comets are 5+inches and the one Fantail is even bigger. Plecostomus I don’t see much but he is about the size of my hand when all spread out. They have no health issues. Tail fins are perfect, no visible parasites, no apparent swim bladder issues.

    Conclusion: Comets and Fantail goldfish can do well in relatively high pH conditions with good aeration, filtration, UV water treatment and beneficial bacteria addition. And with water temps reaching mid 80’sF in the Arizona sun.
    So if you have high pH, do everything you can to have all other water conditions in control for a clear clean pond with a low to reasonable fish count ( I have about 50 gal/fish) and at least for me the fish look fine and not stressed.

  13. my well water comes out at around 7 and was swinging like crazy causing my koi to flash. after finding my kh was low, i added baking soda as instructed by many researches ive done raiding it 20 ppm a day till i reached about 180 kh. now my ph is at 8.2. my problem is, now that my ph is higher, how can i safely add water when needed with my well water being so low in ph?

    • my problem being, now that i raised my kh to a safe level, which raided my ph to 8.2, how can i safely add water for top offs or water changes when my well comes out at a ph of 7….wouldnt that terrorize my koi?

  14. I have a small koi pond and my water test strips are reading a high pH of around 9, alkalinity between 180 to 300 ppm and hardness of less than 20 ppm. I have been trying to research what to do and am very confused as to If I should add baking soda to raise the hardness level or vinegar to lower the pH. Please advise!

    • Hi Martha,

      You need to be careful about raising water hardness as, depending on what compound you use, it can also contribute to an increase in alkalinity. Whereas high/low general hardness is not often dangerous to fish, a high alkalinity parameter can be very dangerous, especially if the parameter increases in a short amount of time.

      Your pH is a little on the high side, but this could just be a normal reading and a part of your ponds daily pH cycle. Even if you don’t have plants, the millions of natural microscopic plants living in your pond will take-in CO2 during the day and increase water pH (due to a reduction of carbonic acid), and then expire during the night, releasing CO2 back into the system and lowing water pH (due to an increase of carbonic acid). Due to this natural cycle of respiration, if you perform a pH test early in the morning, it may present a much lower pH parameter than towards the end of the day.

      In your situation, It may be worth taking 2 readings every day, morning and night, and noting down the parameters over a week period. Calculate the average for the entire week, and note any odd results or higher swings.

      If you can get back to me with your readings I’d be happy to try to advise further! This is a complex subject, and there isn’t always a one fits all solution. I’ll link below some more in-depth articles about alkalinity and general hardness which may provide some additionally light on the subject:

      I’m also in the process of completely re-writing this article to make things much more extensive and clear, as I feel it’s a quite short and certainly doesn’t cover all the inner (most interesting!) aspects of this subject.

  15. We’re close to a wildfire, and smothered in smoke & ash from our fire and other California fires. My goldfish in our outdoor pond suddenly disappeared, then 3 of 5 appeared but don’t want to surface, and are sluggish. Can ash affect water quality? It’s clear but the filter keeps blocking with brown or green algae.

    • Hi GrassValleyMe,

      Thanks for reading! Ash can absolutely affect water quality! Ash, particularly wood ash, is high in nutrients like calcium, magnesium, and phosphorous. So that much ash being deposited by such large-scale fires are likely overloading the pond with nutrients. Combine that (the calcium in particular raises pH) with ash having a high pH, and your pond has likely experienced a spike in pH that your fish are having a hard time with. The algae clogging your filter is likely from the nutrient deposition caused by the ash. I would guess that dissolved oxygen is lower, too, and would very much cause your fish to be sluggish. Just keep doing your best with cleaning the filters, aerating your pond with things like an aerator pump, plants, and bubblers or fountains, and hopefully things improve for you soon! I’m not sure how much it’ll help, but you could also try covering your pond with a shade sail or something similar…this will, of course, block sunlight, but maybe it’ll also help block some of the ash from getting in.

      Best of luck, and stay safe!

  16. I have a 500 gallon pond with goldfish and water lily, hyacinth, and lotus. My KH = 250
    Gh =179
    Ph =9. My lily no longer blooms and is barely growing. How can I lower my PH? (All other readings are 0)

  17. Hi there
    Thank you for the very useful article.

    The PH in my 5000 gallon pond keeps ending up at 9. I added white vinegar a month ago (it took it down to 8.5), and two weeks later added pHdown (also down to 8.5) KH is usually around 180, GH is 160, ammonia is zero, nitrates are zero or close to it. The water is 50°.

    I have read that pH adjustment products can affect beneficial bacteria. Given that my fish are flashing and jumping, I want to get some KoiZyme in, so my question is how soon after finishing pH down can I put in the KZ without it being neutralized by the pH?

    We’ve hardly had any rain, And there are some leaves in the pond but probably not enough to account for the high pH. I fear that the rocks installed by the pond company might be limestone and that is what’s causing this. Ugh!

    This pond has been on a roller coaster this year, being less than a year old. It seemed to stabilize later in the summer…

  18. Hi, please I need your help here.
    I have a pond I constructed late last year but the PH is still at the high side of 9.0 after a whole lot of treatment.

    How can I reduce the lime concentration of the concrete pond.

    My water PH from well is 6.0

    Thanks in anticipation for your quick response

  19. Hello I’m experiencing a couple fish a week dying. This happened late last summer and started a few weeks ago this year. My nitrates are fine along with GH, my KH looks around 80-100, my ph looks to be between 7.5-8. Any suggestions?

  20. Hi, I have pond size 4m3 with water parameters below.
    1. PH8.5 morning and PH8.4 late afternoon
    2. Alkalinity 220ppm
    3. Salinity 3ppt
    4. Ammonia 0 and nitrite 2ppm
    5. Fish starting slow movement and fed.
    Kindly have your advise a few point below?
    1. How to lower PH 7.5-8.0?
    2. How remove off nitrite?
    Best Regards,

  21. Great article! Maintaining proper acidity levels in water is a must. My pet fish needs just the right amount to survive and live healthily. Unstable levels of acidity, whether too high or too low, may bring harm to my pet fish.

  22. Hello, I have a small pond. Preformed. Its about 190 gallons. Its been going for about 6 mths. I have goldfish in it. The water quality has been great until now.
    I am on well water. Its now winter here in Oregon. Its cold, rainy and windy.
    I have a partial cover over my pond to keep the rain out of the pond water. Also a shade sail over that. It still gets some light. It has netting as well.
    I tested the water. Dipped a test strip. Here is the readings i just got.

    Nitrate 100 ppm
    Nitrite. 0 ppm
    Gen Hardness. 25cacc3, ppm
    Free Chlorine. Between 0 and 0.5
    Total alkalinity. 360 ppm
    Carbonate 120ppm
    PH. Between 7.6 and 8.2

    How do i fix this?
    Do i do a partial water change?
    Also i have pond filration, pump, fountain etc.
    Do i treat with something.
    I dont want my fish to die.
    I cant tell if their acting different. Its winter. Torpor. So they hang at the bottom right now.
    Thank You!


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