Water quality is an extremely important aspect of freshwater husbandry, with numerous parameters, measurements, and “ideal” standards we continuously strive to achieve so our koi are able to flourish in their environment. Most fish keepers will likely have a good understanding of certain topics of water quality, such as the importance of beneficial bacteria in regards to ammonia and nitrate control. However, there are other elements of that also contribute to water quality and fish health that can be considered just as important, but sadly tend to fall to the wayside due to confusing definitions and misinformation. Those water quality parameters are that of alkalinity (KH) and general hardness (GH) – both very important aspects of optimal koi keeping.
These two parameters are often interrelated, but are still individually distinct measurements that can have heavy influence on other aspects of water quality, such as pH, ammonia toxicity, and the nitrogen cycle. Alkalinity, or KH ( carbonate hardness), is a measure of waters ability to resist changes in pH with the addition of acids, also known as it’s buffering capacity. Water hardness, or general hardness (GH), is a measure of the total dissolved mineral content in water, often in the form of calcium (CA++) and magnesium (Mg++) ions. General hardness, when caused by calcium, is important for replenishing alkalinity levels, locking down dissolved toxins, and will aid koi in several bodily processes, such as the regulation of salt within the blood.
Both alkalinity and general hardness levels usually come from limestone or dolomite, in the form of calcium carbonates and magnesium carbonates. As it rains, water (H2O) reacts with atmospheric carbon dioxide (CO2) to form carbonic acid (H2CO3), which then works to react with mineral-rich rock to form various metal carbonates. The resulting water is both considered harder due to the dissolved calcium and magnesium, but also higher in alkalinity due to the carbonate component, which works to neutralize acids and maintain stable pH levels.
Pond Alkalinity & KH (Carbonate Hardness) – What’s the Difference?
The total amount of base present in pond water defines the water quality parameter of alkalinity. It is essentially a measure of the waters ability to neutralize acid and buffer pH, also known as its “buffering capacity”, and is formed primarily of carbonate compounds in many hard water regions.
The definition for alkalinity can sometimes vary, but in regards to pond aquaculture, it’s usually a measure of the total amount of bases present that have an influence on the pH buffering capacity of water. Outside the pond hobby, such as in saltwater aquaria, it may also refer to the total amount of alkaline compounds within a solution, including alkali metals that do not contribute to the pH buffer (i.e., sodium hydroxide). For all intents and purposes related to fresh water ponds, alkalinity can be described as the pH buffering capacity of water, compromised of mostly calcium carbonate (from limestone) and magnesium carbonate (from dolomite). It is expressed in mg/L or ppm (parts per millions) calcium carbonate [CaCO3].
KH, also known as “carbonate hardness“, is a term used commonly in the pond (koi) hobby to refer to the total amount of carbonates and bicarbonates within a solution. Although occasionally defined differently to alkalinity, almost all pond testing kits will present KH and alkalinity in the same way, being a measurement of the waters ability to buffer pH and neutralize acid. KH is sometimes refereed to as carbonate hardness, which makes reference to the “carbonate” component of calcium and magnesium carbonate, as these are the primary pH buffering components in many hard water areas.
When testing in fresh water applications, KH and alkalinity can be thought of as the same parameter, although sometimes KH will be presented in degrees (dKH) rather mg/L or ppm. One “dKH” equates to 17.9 mg/L (ppm), which you can use for conversion purposes if necessary.
Note: Carbonate hardness actually has nothing to do with water hardness, so the name can be confusing. It should not be confused with general hardness, which actually does have everything to do with hardness! More info here:- General Hardness Guide.
Why is Alkalinity So Important? How Does it Affect pH & Fish?
Alkalinity (KH) is a requirement for keeping stable pH parameters, and avoiding sudden pH swings throughout the day. The higher the alkalinity in your pond, the more acid can be buffered on a daily basis, and the more stable the overall water pH will remain. In ponds with moderate to high alkalinity, water pH will often be neutral (7.0) to slightly basic, as both carbonates and bicarbonates are bases. Without a sufficient level of alkalinity, your pond would experience huge swings in pH within a 24 hour cycle, with higher pH in the day and lower pH in the night.
These swings in pH are primarily caused by a combination of photosynthesis and respiration from filamentous algae, phytoplankton, and to a lesser extent, fish and bacteria. During the day, plants will carry out photosynthesis and absorb carbon dioxide from the water, which in turn, reduces the amount of carbonic acid and causes a rise in pH levels. During the night, plants will switch to respiration, consuming oxygen from the water and releasing carbon dioxide, causing a return of carbonic acid and a drop in pH levels. Carbonic acid (H2CO3) is a combination of carbon dioxide (Co2) and water (h20), and is the primary contributing factor to pH changes in fresh water ponds.
On top of its important pH stabilization role, organisms such as plants, bacteria, and fish all require a certain level of mineral carbonate to carry out essential biological processes. Beneficial bacteria living throughout your pond and filter will consume a large quantity of carbonates during the de-nitrification process while breaking down waste. Likewise, fresh water fish require a certain amount of calcium carbonate and magnesium carbonate for healthy bodily function, particularly for the regulation of salts within the blood; a process called osmoregulatoion.
In ponds, calcium carbonate (CaCO3) can be considered the most important component of both general hardness and alkalinity. This is due to it’s high pH buffering ability and benefits to bacteria (carbonate-part), as well as its role in the regulation of fish health (calcium-part).
How to Test Alkalinity & KH in Koi Ponds – What is the Ideal KH Level?
- Contains one (1) API POND MASTER TEST KIT Pond Water Test Kit 500-Test, including 6 bottles of testing solution, 3 color cards and 4 test...
- Helps monitor water quality and prevent invisible water problems that can be harmful to fish
- Measures ornamental pond pH balance, Ammonia content, Nitrite levels and Phosphate content
Measurements for alkalinity and general hardness results are often expressed as mg/L (ppm) calcium carbonate [CaCO3], which includes all calcium carbonates, magnesium carbonates, and trace alkali metals. If you live in a limestone heavy region, you can safely assume the majority of your water hardness is caused by calcium, as calcium carbonate [CaCO3].
As calcium carbonate is so important to both pH buffering (the “carbonate”) and fish health (the “calcium”), you can also choose to test your water for just free calcium ions with a specialised water testing kit, such as for salt water reef tanks. These tests can be useful for determining just how much calcium is contributing to general hardness, as you can still have hard water without any calcium carbonate. In regions where hardness is caused by minerals other than limestone and dolomite, the water may be almost devoid of calcium carbonate and magnesium carbonate – the two most important KH buffers for koi ponds.
For ponds without fish, a minimum alkalinity (KH) of 20 mg/L (ppm) [CaCO3] is recommended for good pond productivity. Any lower than this and plants will struggle to grow, phytoplankton will be mostly absent, and beneficial bacteria will suffer without adequate carbonates for the process of waste breakdown. (1)
In koi ponds, an alkalinity (KH) range of 75 to 200 mg/L (ppm) CaCO3 is considered ideal, as water within these parameters will have good buffering capacity against pH fluctuations, and fish will have sufficient carbonate salts for healthy biological processes (1). Where hardness is caused by limestone, this alkalinity (KH) range equates to a general hardness (GH) of 75 to 200 mg/L (ppm) CaCO3.
The table below shows approximate free calcium water concentrations converted to alkalinity (KH) parameters in mg/L (ppm) and degrees of hardness (dKH) with green being the optimal average range for freshwater fish, including koi carp.
Ideal Koi KH/Alkalinity Levels Table
| Free Calcium [Ca++] (mg/L) (ppm) | Alkalinity/KH [CaCO3] (mg/L) (ppm) | Degrees of Hardness [10 mg CaO/L] (°dHk) |
10 25 50 75 100 125 | 25 65 130 195 250 315 | 1.4 3.5 7 11 14 17.5 |
Is it safe to put a cuttlebone in an inside 55 gallon koi tank?
Hi Becky,
Unless your water hardness was already incredibly high, I don’t think it would be a problem. Could I ask, though – is this a growout tank? Also, what is your main goal with adding cuttlebone?
My KH is a little high at the moment in my 5000 gallon koi pond, how can i bring this level down to ideal conditions for the fish??? Thanks Ian
Hi Ian,
Just how high is your KH parameter exactly? If your KH is only a little too high, you could simply remove some water (5-10% ~) via a water change and re-fill the pond with softer water or distilled water. This would effectively remove a portion of the dissolved carbonates that make up the KH parameter, and the new water, which would be lacking in carbonates, would stop the KH from returning back to it’s previous point.
However, unless your KH level is incredibly high, I’d say just leave things as they are and continue to monitor overall water quality. A slightly high KH reading isn’t dangerous to fish, and it will likely reduce by itself in future as the waters buffering capacity (carbonates) are used up through natural processes (rainfall, beneficial bacteria etc.).
To raise both alkalinity and calcium hardness can I use the same stuff that’s made for pools? Or is that not safe for ponds?
Hi Jacqueline,
Could you let me know which product/s you’re referring to?
I have been having a problem keeping my PH down in my 6000 gallon Koi poind. It is fairly new so we are in the throes of trying to get everything in balance.
I am on well water. It is quite hard. GH 32 degrees, KH 20-22 degrees, when I measure the PH of the water directly out of the well it’s PH is 7.6. But after filling my pond my my water consistently shoots up to 8.4 to 8.5 ph with in a few days. I have been using Pool PH Reducer to bring the ph down. This is in the form of CONCENTRATED Granular Sodium Bisulfate. I have now brought the KH levels down to 6-7 degrees of hardness and my ph has dropped to ph 8.1. But that is still pretty high.
GH has gone down slightly to 30 degrees. But that is not expected move with ph reducer and I don’t think I am supposed to worry too much about the GH number in general. But I don’t know how hard water high GH and drastically reduced KH might fight with each other or if it matters at all.
I would like to bring the PH down to 7.5-8.0. Basically the primary question is how low can I take my KH and still feel I have a sufficient buffer to avoid dramatic PH swings. by the way daily PH readings seems pretty sticky I don’t seem to get a lot of swings from morning to evening which is good. If I do take KH to low it is pretty easy to bring it up with a series of water changes. As my source water is lousy with lots and lots of KH (20 degrees as I mentioned above).
Follow-up question, should I be using a different product do reduce PH (muriatic acid?). Something I have not heard of.
Do I need to consider putting in a softener to bring the GH down is is that even a factor. Also softener will then be trading GH for salts, which may not be good for the Koi either, I remember reading about that someplace.
By the way is the product I have been using to reduce PH.
https://www.amazon.com/gp/product/B00YHII1HU
Currently the fish seem a bit stressed at 8.16 ph (they were seriously stressed when any time it approached 8.5 lots and lots of flashing). Currently they like to hang at the bottom of the pond and they flash a bit more then I think they should. Well sometimes it seems like flashing and sometimes it just fells like they are ‘spooked’. But they certainly seem to be behaving a big more frantic then say 3 weeks ago. Previously they would come directly too you looking for food whenever you approached the pond. Now they tend to just hang out near the bottom and jump, often away from me, in a school when I am near the pond. These are 7 one year old Koi and 2 two year old koi. All quite small maybe 5 to 8 inches. All Japanese Koi if it matters.
All the other numbers for ammonia, nitrate, and nitrate, and phosphate all test out at 0.
Sorry there is a lot of information in here but water chemistry is *hard*. I and wanted to give you the most information I can to see if someone can help.
Hi Jeff,
Thank you for the comment. I’ll do my best to give you a hand here!
Firstly, before I mention anything else, I want to emphasize that stability is probably the single most important factor when it comes to keeping koi comfortable and healthy. So long as pH/KH parameters are not reaching extremes, and also don’t fluctuate too much daily, keeping these parameters at a rock-solid level (even if they’re a little high/low) is MUCH better than a constantly changing the underlying water chemistry. I’ll also say that a pH between 7.6 and 8.5 is still perfectly fine for koi. Sure, the upper level is higher than what most would recommend in an ideal world, but I can assure you from personal experience, it’s much better to have a slightly higher pH value which remains constant than it is to have one that fluctuates daily.
Ok, with that out of the way, let’s take a look at what I feel is the main problem here.
In regards to the changes you’re seeing from a lower pH at source to a higher pH in the pond – this is actually fairly typical, especially when using well waters. My guess would be there is a large amount of dissolved CO2 in the well water, and it’s simply escaping (“de-gassing”) upon reaching the more aerated conditions of the pond. The issue here is that dissolved CO2 is acidic, but once it de-gasses the water will become more basic as the acid leaves the system, driving pH upwards as a result. My gut is telling me that it is this pH change after adding fresh well water that is causing your problems with fish stress.
Luckily, this shouldn’t be too difficult to rectify, and I see two fairly straight forward solutions:-
1) Aerate the well (and pond – always helpful!) directly with an electric air compressor. The disturbance in the well will keep the water healthier, and will also displace a large amount of gases before they can be added to the pond. Direct aeration is probably the easiest and fastest way to remedy those pH discrepancies you’re seeing.
2) Pump water from the well but go for an atomized spray delivery method. This will be slower, and you’d need to invest in some equipment, but it would ensure gases escape into the atmosphere before reaching the pond water.
After implementing the above solutions (or similar), here is what I’d do alongside:
1) Test overall water quality (ammonia, nitrates, pH etc.) daily for 2-3 weeks without performing water changes while you get the well water in-check. This will help the natural ecosystem become established, and prevent the swings that are obviously causing koi to become stressed. Readings of 0 nitrogen compounds (ammonias/nitrates) may not actually be good, as this may indicate there is very little for beneficial bacteria to consume and process. Beneficial bacteria needs to establish, spread, and thrive – and to do this it needs time, carbonates/alakinity, and food substances.
2) Continue to feed as normal (ensure it’s a high quality food), maintain filtration/equipment, and just let the koi settle down.
3) After this period, assuming all parameters are holding strong (no big spikes/drops), perform a regular water change with the new well water and treat the pond for parasites; in particular flukes. Winter is coming and parasites are a very common cause for flashing, rubbing, and odd behaviour in general. You could do this first, but unless you’re certain of parasites, I’d let the koi settle down a bit first and sort the well water issue before adding even more chemicals that may cause swings in water quality.
4) Consider adding a dedicated aerator to the pond to provide more oxygenation, if you don’t have one already.
Here are some resources to aid with the above:
https://pondinformer.com/best-koi-pond-aerator-system/
https://pondinformer.com/best-pond-water-test-kit/
https://pondinformer.com/how-to-treat-koi-flukes/
https://pondinformer.com/best-koi-food/
I hope that helps! I’d be very interested to hear how things turn out after a few more weeks, and I’d be more than happy to provide further input if you have more questions or run into problems. My gut tells me the issue is the well water, and simply aerating it to some degree will make a big difference.
Ps – Yes, comments are reviewed before they’re published. Sadly, I get way too much spam these days, so there is now a small delay while I slowly get through the weeks pending comments!
Jeff, what’s up man. This is a long shot for your reply, but I would love to hear how things eventually turned out and what you’ve learned in the following years as you’ve cultivated your pond and fish. I am also a koi keeper, who’s learned from the ground up. Please reach out.
I have a koi pond. It is quite small – 120 gallons. My pH is a bit low at 6.6. My KH is at the lower end of the range at 3’d, and I was thinking of adding more carbonate. But my GH is already high at about 12’d. Will adding more carbonate do any harm?
Hi David,
Yes, I’d certainly advise bringing that KH (and in turn pH) up a little bit. Adding more calcium carbonate should do the trick, just be careful when dosing as your pond is quite small and you don’t want any rapid swings in water quality.
Although KH and GH are often connected, they’re still very distinct parameters. For example, general hardness isn’t dangerous by itself, but as many of the mineral ions that make up GH also contribute to alkalinity ( rising KH/pH), it should still be monitored. However, as seen with your water quality parameters, you can have both high GH and low KH, which usually indicates a lack of the certain minerals – primarily calcium and magnesium carbonates. Since your pH is also fairly low, it’s safe to assume your water lacks ions with the capacity to contribute to the pH buffer, so adding carbonates to raise the kH (and pH) is a viable and safe solution.
Basically, even if your GH is really high, so long as KH and pH are within safe ranges, further increasing hardness won’t be an issue, as it’s the latter two parameters that will cause the major issues with fish, not the water hardness by itself.
Hope that helps!
What should the CH and KH range be ? My PH is 6.0 to 6.5 and 20 NO3 and NO2 between 1-3
Gallons 900-1000
Carolyn
Hi Carolyn,
Your general water quality parameters look fine. Do you have many plants? Adding more plants will help soak up any extra nitrates (NO3), and performing small weekly (10%~) water changes will help reduce all nitrogen compounds and replenish alkalinity and trace minerals.
Optimal alkalinity (KH) for most types of pond would be between 130-200~ ppm; assuming the KH parameter is comprised primarily of calcium and magnesium carbonates. You can perform a free calcium test to determine this which will check specifically for calcium carbonate ion concentration in water.
Please help, I have a 165 gal. Pond. 2 koi’s 2 goldfish. Maybe 4 ” in size. I did a strip test and my GH is 180 KH 180 as far as I can read these strips. I have a UV light and use a Pond Fresh water filter to fill and add water. Have a few underwater plants and 3 floaters. These strips confuse me to know if these readings are ok. The others readings were about in the middle. Do I need to do anything else to make my fish healthier. Tyvm
Hi Sandra,
Strip tests are not always the most accurate, so if you’re worried the results may be incorrect, you could try a liquid GH/KH test to be sure. You can find an example of such a kit here – https://pondinformer.com/best-pond-water-test-kit/
With that said, assuming your strip test is correct, a GH/KH reading of 180 mg/L (ppm) CaCO3 is perfectly fine (actually, it could be considered ideal!) and there nothing you need to do to correct it.
If you’re worried about any other parameters, let me know which ones and any test result readings you have and I’d be happy to advise if I’m able!
i have a 2,500 koi pond with river falls that run 24/7. GH is 150, kh about 250 and PH about 8. Chlorine & nitrates are great. Any suggestions on bringing down GH, KH and PH?
Hi Michael,
Are your koi showing any signs of stress or sickness? Does your pH fluctuate (outside the normal day/night cycle), or is it fairly stable?
Even though your pH reading could be considered little on the high side, I know for a fact, having a very stable pH parameter is far better than a changing pH, even if the stable reading is a little high/low. Adjusting pH (and by extension KH/alkalinity) is a risky business, as adding too much acid to the system has the potential to exhaust the KH buffer and cause a very dangerous ‘crash’ in pH. Likewise, even if you did drop the KH and pH safely, if your mains water supply simply brings up the KH and pH again during a water change, you’d be introducing a swinging pH value to the pond which will almost certainly stress fish more than a stable one.
If your other water parameters are in good standing, and your fish seem happy and healthy, I’d say just leave things as they are unless you notice the pH climbing further.
Hi we have a new large pond approx 7000 litres and the PH is high at 9.5 and the KH is also showing reading of high 20dh. The water is really green and we have completed several 25% water changes with no effect. We have added lots of plants and have reduced the amount we are feeding the fish. Our Koi keep dying and we have tried everything to turn the water quality around. All other readings are fine. Please help!
Hi Hazel,
I’m sorry to hear about your water quality issues! That pH is very high indeed.
Can I ask, what is the pH/KH of your source water? Does the pond slowly increase in pH over time, or is this the water quality you’re seeing straight from source? What are your other water parameters showing, such as ammonia, nitrites, and nitrates?
I’m having the same issue my water source is from my well kh and ph high gh low everything else is good my pond is most Sun and have plants two aerators skimmer waterfall and a pressure filter with uv light, also cleaning filters out two or three times a day from slimy algae that gets lose I’ve been doing small water changes daily ph has always been high 3000 gallon pond not over crowded. Should I worry ph 9.4 kh 268 Gh 4
Hi Brenda,
That pH is a little high. What kind of fish do you have and how are they growing/behaving? Are your water test results from the well water or the pond water? Dealing with pH in well water can be tricky, and making sudden changes to water chemistry can often cause more problems with fish than leaving things be. If your fish are happy, feeding and growing strong, I don’t think you need to worry too much here.
Hello Chris,could you advise me please as I really don’t know what to do.i have just done a water test using pondlab 200 test kit.
Ph =7/8(in the middle)
Kh = 4 (dh)
Gh = 10 (dh)
Nitrate =2.5
Nitrite = 0/0.25
Ammonia = 0/0.2
I have read that the kh is a problem at this level but my Gh is high so not sure if I should leave alone.i treated the pond last week with an ulcer treatment,enough salt for 3700L.yesterday after doing approx 20-30% water treatment I added a good bacteria treatment to the filter.
I have pond with 6740L and it contains 9 koi of various sizes from 10 “ to 3”
Thank you
I really need help. My wife range PH is perfect during the day, but HIGH at night. One fish died. All other readings (ammonia, nitrite, nitrate) are great. How can i have perfect ammonia readings but high PH? Doesn’t high PH mean ammonia? My fish who died has really red gills…and the other one does too. Did partial water change too. But my issue is it’s spiking at night not crashing.
Hi Sherry,
I’m sorry to hear about your fish loss! Never an easy thing.
Could you let me know your exact pH parameters, both morning and evening? Also, do you happen to know your current KH/Alkalinity parameter? Finally, is your water supply mains or natural/well water?
Although ammonia can become more toxic in high pH waters, the compound itself doesn’t really alter pH to any noticeable degree, so I don’t think the ammonia alone could be the cause here.
I could use some help.
PH = 8.1
Ammonia, Nitrite and Nitrate are at 0
KH = 35ppm
GH =107ppm
I have a 400 gallon fish pond with koi and no vegetation. I have a filter system with a UV light that is running. But we went about 3 weeks without it because the pump had to be replaced and then the filter with the UV light went out, requiring a change to the UV light. After getting all of that fixed the pump and filter have been running constantly for a month. It seems that ever since then the fish have not been coming up to eat. They seem lethargic and stay near the bottom of the pond. We have had warmer weather, but that didn’t seem to bother them in the previous years.
How can I bring up the KH without raising the PH and GH? or do I need to? Any suggestions would be very helpful.
I have a similar situation as Marilyn. My pond is actually a repurposed “kiddy pool”. Just over 500 gallons. All levels seem to be in normal ranges but the Alkaline is low. About 20 ppm. This is new to us. We have natural rock inside and an air pump. The fish tend to hide in the rocks, sticking close to and eating from the bottom. They must be coming up at night to eat the pellets at top. I have one that seems to be eating or pecking up and down the walls for about a week and has become anti social as well. He seems lethargic. Staying in one spot for as much as much as two days! He previously had a lot of energy and stayed with the other 4. All 5 are about 3 to 4 inches long. They’ve been with us about a month now.
I need help my 10,000l koi pond has a scummy/white/foam surface, after I totally cleaned filtration and my levels are all good except KH = 0 and GH = 3
What should I be doing, I’ve heard calcium carbonate but where do I buy that?
Any help appreciated
I just installed a copper ionizer in my koi pond, with the initial setting at 2. After a week, I tested the alkalinity and copper levels. The alkalinity tested at 3 and the copper at 0.4. My reading up on this tells me that the Kh level is too low. I added some baking soda this morning and the Kh is up to 5. I’ve turned the ionizer to “0” for the time being. My koi are sluggish, hiding and not eating. Hopefully I haven’t harmed them. Are my levels ok, or should I increase the Kh, if so to what level?
I have a 6k GAL koi pond, 10 months old, I have 10 koi, pretty small. I’m in direct sun most of the day, lots of plants, aerator, several waterfalls, and the rock is a mix of limestone and sandstone (80/20% estimate). Ultima II Bio Filter rated for a 10k Gal Pond. Pump is at 5.8k GPH
My fish are acting normal, I did I find one dead this morning no signs of trauma.
Anyway, test results at 3:25 pm CDT:
PH 9
Ammonia 0
Nitrites 0
Phosphate 0
KH 125.3ppm
GH 220 ppm (took 13 drops, one level off the chart on my test kit).
Is the GH a problem? What about the PH? It is sometimes lower in the morning (around 7.5-8) and raises during the day.