All About Aquarium KH (What IS It And What’s Best For My Fish)

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KH, also known as carbonate hardness or buffering capacity, is one of the most important water parameters for maintaining a healthy aquarium. Getting to grips with aquarium water chemistry can be tough though, and terms like pH, GH, and KH often leave beginners scratching their heads.

If that sounds familiar, don’t worry, you’ve come to the right place! This simple guide will teach you everything you need to know about KH in aquarium water and how you can adjust it to make your fish stay healthy.

Key Takeaways

  • KH (Also known as carbonate hardness/ buffering capacity) is one of the most important water parameters in a fish tank.
  • Aquarium KH levels buffer the pH of your water and help to avoid pH swings that can harm your fish.
  • KH levels naturally decrease over time, so it’s important to test your tank water regularly.
  • You can raise or lower KH in your aquarium, but it may be better to choose fish that are suited to your natural water parameters if you’re new to the aquarium hobby.

What Is Aquarium KH?

Aquarium KH measures the concentrations of carbonates and bicarbonate ions dissolved in water. These concentrations vary depending on the geology of your area, so your tap water may have different levels to the next state or town.

You can measure and adjust your KH levels at home using products that are easily available at pet and local fish stores. A range of about 4-8 degrees or 70-140 parts per million is generally recommended for most freshwater aquariums, although different fish and plants have different water parameter needs1.

Why Does It Matter?

Minerals are essential for fish health, but they also play a vital role in regulating your aquarium water chemistry. The minerals that determine your KH levels ‘absorb’ natural acids and prevent them from changing your water chemistry.

Author Note: You can think of KH as a sort of safety net for your aquarium. Without the buffering effect of KH, acids in the water can have immediate effects on your water chemistry, and that can be harmful to your fish.

So, the higher your KH, the more potential it has to neutralize or ‘buffer’ acids in the water before they can affect your pH.

What Is The Difference Between KH and GH?

GH (general hardness) is another important water chemistry parameter that is often confused with KH. While KH is a measure of carbonate and bicarbonate ions, GH describes the levels of magnesium ions and calcium ions dissolved in the water.

General water hardness does not have such a direct effect on pH levels, although hard water typically measures higher on the pH scale.

What is pH?


pH (short for ‘Potential of Hydrogen’) is a chemical measure of the acidity or basicity of a liquid. All liquids have a pH level, including your tank water.

The pH scale ranges from 0 to 14. Aquarium water between 0 and 6 on the pH scale is acidic, and from 8 to 14 is basic. Neutral water is right in the middle at 7 on the pH scale.

You can measure pH at home using an aquarium test kit, and it is possible to adjust your pH levels, although it’s better to aim for a stable pH than to chase a specific number on the scale.

Why Does pH matter?

Each fish and aquatic plant species has a preferred pH range, although many species are pretty adaptable as long you can maintain a stable pH.

Some aquarium fish do well in pH levels as low as 5 (discus) and as high as 8+ (African cichlids), but most freshwater fish prefer a stable pH level between roughly 6.5 and 8.

Maintaining a stable pH level can be difficult if your water has a low KH or buffering capacity, and rapid swings in pH can cause major stress on your fish.

What Affects pH Levels?

KH and pH are usually related in the aquarium, so if you have a low KH, you’re likely to have a low pH too. If you’d like to adjust your pH to keep specific fish species, you’re going to need to change your KH first.

pH levels tend to decrease over time, and the change can happen gradually or even suddenly in the case of a pH crash, but what causes these changes in water chemistry? Let’s take a look at four common causes of aquarium pH shifts.


You’re probably familiar with the aquarium nitrogen cycle, and how beneficial bacteria in your filter convert ammonia from fish waste into nitrite and nitrate. Well, nitrites and nitrates are acidic, which means they lower the pH of your water.

The best way to manage the nitrates in a fish tank is by performing regular water changes to remove them from the system, but you can also reduce the build-up by understocking your tank, growing live aquatic plants, and avoiding overfeeding.


Have you ever added a piece of driftwood or some almond leaves to your tank, only to see the water stain yellow or brown?

Tannins are acidic chemical compounds found in plants that can leach into the water, causing the ‘black water’ seen in many tropical freshwater environments. The effect is usually pretty weak, but tannins can reduce the pH in a freshwater aquarium, especially if you have low KH levels.

Carbon Dioxide

Carbon dioxide is acidic, which means it lowers the pH of aquarium water. Many aquarists use pressurized CO2 to increase plant growth, which is perfectly safe as long as the system runs on a timer to switch off at night when plants no longer photosynthesize. During the night, pH levels rise as the CO2 leaves the system.

Substrates and Rockwork

Crushed coral or dolomite substrates and limestone-based rock work like texas holey rock can increase the pH of acidic water.

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How To Test Carbonate Hardness

So now you know why KH is so important for maintaining healthy water chemistry, but how do you manage something you can’t see, touch, or smell?

You can test your water’s KH levels or buffering capacity at home using water test kits. Most strip test kits will measure KH, and these are a good option for testing general water parameters. However, liquid test kits tend to be more accurate, even if they do take a little more effort.

Start by measuring your source water, whether it comes from a tap, a well, or any other source. This will give you a good baseline reading that you can monitor over time.

Now, KH levels tend to decrease over time as acids are released into the water, so you’re going to need to test regularly to find out how long you can go between water changes or treatments. Once a week is a good schedule if you’re starting out with naturally low KH levels.

Suggested Carbonate Hardness

So now that you know what carbonate hardness (KH) is and how to measure it, you’re probably wondering what your levels should be in your tank water. The answer depends on which kind of fish you keep, so keep reading to learn about suggested levels for specific fish types.

  • African Cichlid Tank: about 200 – 400 ppm/ 11-22 dKH
  • Discus Tank: about 50 ppm/ 0-3 dKH
  • Planted Tank: about 50 – 100 ppm/ 3 – 6 dKH
  • Brackish Water Tank: 200 – 400 ppm/ 11 – 22 dKH
  • Koi Pond: about 125 ppm/ 7 dKH
  • Shrimp Tank: 18 ppm or 1 dKH for Caridina shrimp. 54 – 180 ppm/ 3 – 10 dKH for Neocaridina
  • Typically Community Tank: 70 – 140 ppm/ 4 – 8 dKH
  • Saltwater Tank: 140 – 2000 ppm/ 8-12 dKH
Author Note: KH is described either as parts per million (PPM) or degrees (dKH/°KH), and one degree is roughly equivalent to 18 ppm.

The figures mentioned above are good general guidelines, but you should definitely research the preferred KH range of each fish species you keep to make sure you can provide a healthy tank environment. Remember, each fish in a community setup should be comfortable in the same tank.

How To Adjust Levels

Sometimes you need to adjust your KH to keep certain fish species or to increase your tank’s ‘safety net’ against pH swings. Continue reading to learn how.

How to increase carbonate hardness

Water Changes

Carbonate hardness naturally decreases over time as acids are neutralized and carbon escapes the tank in the form of carbon dioxide. Performing regular water changes and sucking up decaying organic matter from the substrate will remove acidic nitrates from your water.

Topping up your tank also reintroduces carbonate and bicarbonate ions to increase your KH levels. However, water changes are not going to increase your carbonate hardness above your source water’s (tap, well, etc.) natural KH level.

So how do you increase KH above the levels of your source water? Continue reading to learn about five things you can add to the water to increase your KH.

Alkaline Buffers

Alkaline buffers, like the range produced by Seachem, make raising and maintaining your KH levels very easy. These products provide excellent dosage instructions to make your adjustments much safer and more precise.

After regular water changes, alkaline buffers are the best option for beginners, and you might even find a product designed specifically for the type of fish you keep.

Crushed Coral

Crushed coral is mostly made up of calcium carbonate, which is great for increasing your water’s buffering capacity.

It is easy to find and easy to use, simply pick up a bag from your local pet store or order it online and mix it in with your gravel at the bottom of the tank. Alternatively, add some to your filter’s media basket in a mesh bag.

Aragonite and Dolomite Substrate

Aragonite is a natural form of calcium carbonate, which is the same mineral that makes up crushed coral. It has a fine, sand-like texture and it makes an ideal substrate for raising KH levels in African Cichlid tanks.

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Dolomite is a mineral composed of calcium magnesium carbonate. It is another popular substrate that raises both the KH and general water hardness of your tank water.

These substrates provide a long-lasting effect, but you can’t remove them without completely draining and re-scaping your tank.

Baking Soda and Soda Ash

Baking soda (sodium bicarbonate) and soda ash (sodium carbonate) are easily available and highly affordable minerals that can be used to raise KH and pH in aquariums.

However, they must be used very carefully since they can cause sudden and powerful swings and may need frequent dosing to maintain stable KH levels. While they can be effective, they are not as safe or easy to use as purpose-made aquarium alkaline buffers.

How to Decrease Carbonate Hardness

Some animals, like Caridina shrimp and discus fish, need very soft water to stay healthy, so what do you do if the KH levels in your source water are too high?

It’s not practical to remove carbonate hardness from your tap water, so your best option may be to cut your high KH water with something with a lower carbonate hardness. If you want to go really low, you may need to switch to a completely different water supply.

Let’s take a look at some of the best options.

Reverse osmosis water

Reverse osmosis (RO) is a process that removes minerals from water by passing it through a membrane, leaving it with practically no carbonate hardness. It is safe for use in aquariums, but only if you add essential minerals to the water using products like Seachem equilibrium and Alkaline Buffers.

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You can also mix RO water with your regular source water to reduce your KH levels, although you’ll need to use your test kit to work out the correct ratio for the type of fish you keep. A 50/50 mix of this pure water and your regular water will halve the KH and GH of your tank water.

Reverse osmosis is often available for sale as drinking water, but using this in your aquarium will become expensive, especially if you have many fish tanks. RO filtration systems have become more affordable and easier to source, so it might make more economic sense to set up your own dedicated system in the long run.

Distilled Water

Distilled water has similar properties to reverse osmosis water but the purification method differs. This water is purified by boiling and collecting the evaporated H2O molecules.

Pure distilled water contains none of the minerals that fish and aquatic plants need for healthy biological functions, so you should not use it without adding minerals or mixing it with tap or well water.

Buying distilled water is a good option for small freshwater tanks, but it will get expensive for larger aquariums.

Organic Matter

You can also decrease your KH levels a little by adding aquasoils, driftwood, and peat moss to your tank, although the effect may last just a few weeks or months.

Reducing the frequency of water changes is another possible option, but you will need to monitor your nitrate levels carefully to prevent any health issues in sensitive fish species.

Acid Buffers

You can also lower the KH levels in your water using purpose-made aquarium products. Acid buffers convert KH into carbon dioxide, which can be great for freshwater aquariums with live aquatic plants.


What is KH in fish tanks?

KH (also known as carbonate hardness or buffering capacity) is the measure of carbonates and bicarbonates dissolved in the water. It is an important water parameter in both saltwater aquariums and freshwater aquariums that stabilizes the pH levels.

Is KH and GH the same thing?

GH (general hardness) and KH (carbonate hardness) are both important water parameters, although they have different effects in a fish tank. General hardness measures the dissolved calcium and magnesium ions, rather than the carbonate and bicarbonate ions in the aquarium water. While KH and GH levels tend to be related, you should always test for both.

Which fish can live in high KH?

Many popular aquarium fish thrive in water with high KH levels. African Cichlids, livebearers like guppies and mollies, goldfish, and brackish water fish are all examples of fish that prefer water with high pH, GH, and KH water parameters.

Final Thoughts

KH is an important water parameter that all aquarists should understand, especially when keeping sensitive fish or species with very specific pH requirements. Fortunately, you don’t have to be a qualified chemist to understand the basics of aquarium water chemistry.

A good quality water test kit and half an hour each week for a quick water change is usually all you need. However, there’s also a range of excellent products available to the modern hobbyist that can make managing high or low aquarium KH levels really easy.

How do you manage the KH levels in your aquarium? Let us know in the comments below!

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