Guide to Koi Pond Alkalinity, KH, & Carbonate Hardness (Ideal levels)

Guide to Koi Pond KH, Alkalinity, & pH Buffering 2018 (Testing & Ideal Levels)

Alkalinity is a measure of a ponds ability to neutralize acids and resist changes in pH – also known as “buffering capacity”.

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? 

Hard water and alkalinity are closed linked, with many ponds alkalinity coming from naturally occurring limestone.

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 Effect pH & Fish?

Plants, algae, and phytoplankton will cause pH to rise and fall throughout the day, with the rate of change determined by total alkalinity.

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.

Calcium, as calcium carbonate, is one of the most important molecules for both water hardness and alkalinity in koi 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?

You can test alkalinity/KH using one of many commercial pond water testing kits designed for general hardness and KH measurements. In many cases, you should not be surprised if you notice the test results for general hardness (GH) are similar, if not identical, to the KH and alkalinity results. This is because limestone (and dolomite) commonly make up the largest majority of water hardness, being comprised mainly of calcium carbonate [CaCO3] (2/3) and magnesium carbonates [MgCO3] (1/3). In these instances, the calcium and magnesium make up the “hardness” (GH) parameter, and the carbonate makes up the alkalinity (KH) parameter – but they’re still weighed as a single molecule, hence the similar results.

Water test kits designed for reefs are useful as they provide measurements for both calcium hardness and KH/alkalinity.

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.

Free Calcium [Ca++] (mg/L) (ppm)Alkalinity/KH [CaCO3] (mg/L) (ppm)Degrees of Hardness [10 mg CaO/L] (°dHk)



















Pond Informer (Chris)

Pond consultant and long-time hobbyist who enjoys writing in his spare time and sharing knowledge with other passionate pond owners.

6 thoughts on “Guide to Koi Pond Alkalinity, KH, & Carbonate Hardness (Ideal levels)

    • 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?

  1. 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.).

  2. 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?

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