How to Lower Pond pH (Advice & Best methods)

Help Spread Pond Keeping Knowledge!

Why is my Pond pH so High? Guide to Lowering & Stabilizing Water pH (Updated)

koi pond ph
Pond fish, such as Koi, are particularly sensitive to changes in water pH

Water pH, or “potential for hydrogen”, is a measurement scale used to determine the acidity or basicity of a liquid solution. The scale ranges from 1 to 14, with 7.0 being considered neutral. In regards to garden ponds, a safe pH range would be between 6.0 to 8.5, but a range of 6.8 to 7.8 would be even more ideal for ponds with fish (especially koi).

The only way to know your pond pH is to physically test the water using a testing kit or an electronic reader, but this is typically only necessary if you feel there is a problem with water quality. Most ponds will naturally fall within the safe range and never see any large spikes or drops in pH, but you should still be on the look out for tell tale symptoms! For example, if fish become sluggish, sick, or start to die, this could mean there has been a rapid change in pH which needs to be checked. Other symptoms include sudden changes in water clarity, odor, or an overgrowth of algae.

An increasing pH is very dangerous to pond fish, as it directly affects the toxicity of a waste chemical called ammonia, with rising pH making it far more potent. With that said, the pH of a pond should never be rapidly adjusted with chemicals as this can also be very dangerous to fish. Daily pH fluctuations should be no more than 0.8 a day for fish ponds, and a stable pH within the “safe range” is always more ideal than a constantly changing value.

What causes high pH in ponds?

Algae is just one of many reasons your pond may see a change in pH as it removes CO2 from water.

A rising or high water pH in a pond is much more common than a lower pH. The higher pH means the water is becoming alkaline, and the cause for the rise is often substances leaking into the water from the ponds surroundings. A higher water pH can become especially dangerous for pond fish, as both high pH and high temperatures will increase the toxicity of ammonia in water. Ammonia is already toxic to fish, but the bulk of ammonia is generally removed through normal filtration by beneficial pond bacteria. Problems occur when both pH and temperatures spike, and the potency of ammonia increases two-fold. In situations such as this, fish will rapidly become sick and can even die within days.

The most common cause of increasing water pH is from the leaking of toxic lime from uncured cement pond embankments. If you have a cement pond, the cement should be properly treated before installation to prevent substances leaking into water. Often cement is not treated, however, and there is a gradual increase in lime leaking into the water causing pH levels to rise.

This same scenario can occur from bricks, mortar, or ornaments around the pond, and is not just limited to cement ponds. As it rains, untreated materials may slowly leak chemicals into pond water which will begin to alter the pH of the water.

Another common reason for a rising pH is from the rapid growth of algae. During the day algae will use sunlight and carbon dioxide for photosynthesis, and the removal of large amounts of carbon dioxide can cause an increase in water pH level and drop in acidity. Algae should be maximally treated if you have a bloom before you begin adjusting pH values.

Testing Water for pH and KH

Basic test strips can provide results for both KH and pH

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 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

1) Stabilize Water KH (Prevents pH Spikes)

KH boosters are an easy way to raise water KH and stablize pH

If you sudden experience spikes in pH or a gradually rising/falling pH reading, you should check the KH (carbonate hardness) of your pond. KH is essentially an indicator of the number of dissolved bicarbonate and carbonate ions in the water which work as a buffer, preventing rapid changes in pH. The ideal KH range is between 125 and 200 ppm, and a result much lower than this range is likely the culprit causing the swing in water pH.

Since rapid changes in pH are very dangerous, the best way to start lowering your pH would be to first stabilize your KH levels in water. We recommend rising KH levels if KH drops below 125 ppm or you see pH changes above 0.8 points each day.

To raise the KH of water you can simply add sodium bicarbonate (baking soda) to the water daily until KH begins to rise, or you can treat the water with a dedicated KH Booster product. Both methods will work to increase water KH, but you will need to first test for KH beforehand so you have a starting point. For example, if your KH is 100 ppm, you will want to raise your KH by a further 25-50 ppm, so you will need to measure out the correct dose to use. For booster products, you can simply follow the labelled instructions for best results. If you’re using baking soda, a rough guide is below on the amount required to raise KH:

Pond Size (US gallons)Rise by 30 ppmRise by 40 ppmRise by 50 ppm
 100 gallons

250 gallons

500 gallons

1000 gallons

0.7 oz

1.8 oz

3.6 oz

7.2 oz

 1 oz

2.4 oz

4.8 oz

9.6 oz

1.2 oz

3 oz

6 oz

18 oz


2) Remove Algae from Pond

UV clarifiers are a safe and effective way to remove and control algae

An algae bloom is a common cause for high pH during the day as algae consumes carbon dioxide and other inorganic compounds for photosynthesis. If your pond looks very green in color, an increase in pH and rapid changes of pH between morning and night is very likely caused by algae. Before trying to adjust the pH of water, the maximum amount of algae should first be removed from the pond.

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.


3) Increase Acidity of Water

pHDown contains naturally occuring acids which help lower pond pH gradually

After your water KH is in the range of 125 – 200 ppm and algae is 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 – 8.5 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.

If your KH is stable but your pH is above 8.5 and you want to lower it, we suggest 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 lowing 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 PondCare’s pHDown 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 pH needs to be lowered further, a repeat treatment can be applied to the pond.

27 thoughts on “How to Lower Pond pH (Advice & Best methods)”

    • 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 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?

  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 – https://pondinformer.com/best-small-pond-fish/

  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.

  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.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.