How To Lower KH in Aquariums – Our Informative Guide

Alison Page

Alison Page


how to lower kh in aquarium

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The KH levels in your fish tank are directly linked to the health and well-being of your fish and other livestock. Sometimes, those levels become out of kilter, being too low or too high, which can affect other crucial water quality factors, such as pH.

But why is the correct KH level important? How do you raise and lower the KH level in your fish tank? And how can you check the KH levels in your aquarium?

Keep reading to learn everything you need to know about your fish tank’s water chemistry and KH levels!

What Is Aquarium Water KH?

KH stands for carbonate hardness and refers to the measurement of bicarbonates (HCO3) and carbonates (CO3) dissolved in the water.

KH essentially helps to keep your water pH levels stable by buffering the pH level against acids. So, the pH level remains constant while the KH level is stable. However, if the KH gets too low, the pH might begin fluctuating, which can be bad news for your fish.

When KH levels in the water are high, more acid is neutralized before it affects your pH.

KH is also sometimes referred to as the “alkalinity” of water. However, you mustn’t get alkalinity confused with alkaline.

So, what’s the difference?

  • Alkalinity is the measurement of acid neutralization (KH).
  • Alkaline is at the opposite end of the pH scale to acidic conditions and is sometimes also referred to as basic.

KH can also be referred to as:

  • Temporary hardness
  • Carbonate hardness
  • Buffering capacity/buffer
  • Total alkalinity
  • Acid-neutralizing capacity (ANC)

All these terms are interchangeable and are essentially the same thing.

Are GH and KH The Same Thing?

Newbies to the hobby often confuse carbonate hardness (KH) with general hardness (GH). However, these measurements refer to different water parameters, and they are not the same thing.

Carbonate Hardness (KH)

KH refers to the levels of bicarbonates and carbonates dissolved in your water.

General Hardness (GH)

GH refers to the levels of calcium and magnesium dissolved in your fish tank water.

What About Tap Water?

In the natural environment, KH and GH are closely related. So, if a water body has a high GH level, it will also have a high KH.

When someone refers to having “hard water” in their area, they are talking about GH. Tap water can have a very low KH but an excessively high GH. So, because your water has a high GH, it doesn’t automatically follow that the KH will also be high.

What’s So Important About KH In Your Fish Tank?

As previously mentioned, KH can prevent acids from causing fluctuations in pH. Fluctuations in pH levels can shock your fish and is sometimes responsible for dead fish in your tank.

Acids are constantly produced in the aquarium. How so?

Well, the beneficial bacteria that live in your biological filter media are the culprits for those high acid levels!

Organic waste, such as fish waste, plant debris, and leftover fish food, gradually decomposes, producing ammonia. The bacteria in the biological filter convert the ammonia to nitrite and then to nitrate.

Both nitrite and nitrate are acidic. Since those chemicals are constantly being produced in your aquarium, the water’s pH will gradually decrease. If the KH in your tank is low, nothing in the water will neutralize the acids. That can cause pH levels to fall, creating a toxic environment for your fish and plants.

KH prevents that from happening, so you need to maintain a constant level of KH within your aquarium, regardless of what livestock you have.

If you have a marine tank, KH is vital for the health of corals. Corals use carbonate to build and maintain their exoskeletons. So, if you’re building a reef tank, you must carefully monitor the KH.

What KH Level Does My Tank Need?

The perfect KH level for your tank depends entirely on what livestock and plants you keep.

The levels shown in the table below are listed as degrees of carbonate hardness, i.e., dKH, which equates to 17.9 ppm (parts per million). The recommendations shown are intended as general guidelines only.

Aquarium KH Levels

Aquarium Type
Tropical4 to 8 dKH
Shrimp tank2 to 5 dKH
African Cichlid10 to 18 dKH
Discus3 to 8 dKH
Planted tank3 to 8 dKH
Brackish environment10 to 18 dKH
Pond Habitat4 to 8 dKH
Marine Saltwater Aquarium8 to 12 dKH
Reef Tank8 to 12 dKH

Note that your livestock and plants might require more specific KH levels, so we recommend that you research them thoroughly.

Will Increasing KH Also Raise The pH?

When the KH is increased, more acid in the water is neutralized, so the pH generally remains within a higher range.

However, it’s more important that the pH remains stable even if it’s slightly higher than ideal rather than constantly fluctuating.

How Can I Test The KH In My Fish Tank?

To determine the KH levels in your fish tank, you should use an aquarium test kit that’s specifically formulated to test KH. Although you can buy testing strips, we recommend using a liquid kit since they tend to produce more accurate results.

The frequency of water testing depends on several factors:

  • You should test the water weekly in a freshwater tank where the dKH is 4 or lower.
  • In a freshwater aquarium, where the dKH is 5 dKH or higher, check the KH monthly.
  • In saltwater setups, you should check your KH every week.

It’s helpful to test your tap water, too. That will give you a greater understanding of how to adjust your KH if necessary.

If you have a saltwater tank, we recommend testing the KH of your salt mix.

How To Lower KH In Your Aquarium

As previously mentioned, your water KH can prevent your pH from dropping, which is essential for fish that are very sensitive to the water chemistry in their environment.

But sometimes, the pH of your tap or aquarium water is too high for your livestock and plants, so you’ll need to take steps to lower it. To do that, you’ll need to lower your water KH.

When lowering your KH, you need to strike a balance between that and your pH. Otherwise, you risk causing dramatic fluctuations in pH that could kill your fish.

Note that decreasing the water pH is only really practical in freshwater tanks.

Acid Buffers

Basically, acid buffers convert KH into carbon dioxide (CO2). That reduces KH levels and lowers pH.

If you decide to use an acid buffer, remember to take it slowly. Acid buffers are mostly used in tanks with aquatic plants. In planted tanks, the plants remove excess carbon dioxide from the water, helping to maintain a healthy balance.

However, if you have a tank with no live plants, using too much acid buffer can cause massive drops in pH and excess CO2. That can quickly be fatal to your fish!

So, be sure to follow the product manufacturer’s instructions carefully and gradually increase the levels of acid buffer you use to avoid overdosing your tank.

RO/DI Water

You can make pure water by using one of the following:

  • Reverse Osmosis (RO) system
  • Deionization (DI) filter

Both these systems create water that has no KH. Although setting these units up can be quite expensive, you can save lots of money in the long term, especially if you’re buying multiple bottles of distilled water for your fish tank every week.

When using RO/DI water for your fish tank, you’ll need to mix the water with tap water to ensure that there’s a small amount of KH and GH present. So, you can balance and adjust the amount of KH in your water by using more or less RO/DI water.

Marine or reef aquarists generally use an RO/DI unit routinely, and this solution is a popular choice for hobbyists with larger tanks.

Distilled Water

Distilled water is essentially pure water that contains no KH or impurities. Distilled water is created by heating water until it turns to steam. The steam passes through a cooler and is collected in a separate vessel. Any impurities, minerals, heavy metals, etc., are left behind, leaving only pure water.

If you don’t fancy making your own distilled water, you can buy it in gallon containers from your local grocery store.

Again, you need to combine distilled water with tap water since you still want to have some GH and KH for the health of your fish and plants. So, you can add 50% tap water to your distilled water using your aquarium water test kit to determine the correct ratio and KH levels.

Expensive Solution

The main drawback to using distilled water for your fish tank is that it works out pretty expensive, especially if you have a large aquarium. However, a distilled water and tap water combo can work well if you own a nano tank.

Indian Almond Leaf

You can use Indian Almond leaves to lower the pH and KH in freshwater fish tanks. This solution is especially useful if you have fish species that prefer a blackwater river ecosystem or if you keep a betta tank.

As the leaves decompose in the water, they produce tannic acid that breaks down the KH. As an additional bonus, Indian Almond leaves are also said to have medicinal properties, helping to protect your fish from bacteria and aiding wound healing.

The main problem with using Indian Almond leaves is that they act slowly and gently. So, if your water KH is very high, you probably won’t see much of a difference.

Peat Moss

Peat moss can be placed inside a mesh bag or a clean stocking and added to your filter media.

In the same way as Indian Almond leaves, peat moss leaches tannins into the water, reducing the KH and pH slightly. Again, peat moss is best used where you only want a small reduction in the water KH.

Note: Be sure to buy aquarium-safe peat moss rather than the peat you’ll find in your garden center. That kind of peat contains chemicals to kill mold that would harm plants and can make the water toxic for your fish.

Can Your KH Levels Ever Be Zero?

how to lower kh in aquarium

Your fish tank should always contain some levels of KH, regardless of what livestock and plants you have.

A zero pH is dangerous in an aquarium because it can lead to a large drop in pH, creating a toxic environment for both plants and fish.

You’ll usually see articles recommending zero pH for certain species of shrimp and some fish, such as discus. Although it’s true to say that some fish live in a habitat where the natural water KH is close to zero. However, those water bodies have many other variables that stabilize the pH levels.

We don’t recommend creating an environment with a zero KH level. Fish require a stable pH to thrive and live to the upper limit of their lifespan; having some KH in the water goes a long way to ensure that.


Here are the answers to a few of the most commonly asked questions about aquarium KH.

Q: Will Increasing My KH Also Raise My pH?

A: Aquarium pH levels tend to fall over time. When the KH is increased, more acid in the water is neutralized, so the pH tends to remain within a higher range.

For example, if your tank has a higher pH level of 8.0 and a buffering agent is added, such as crushed coral, the KH will rise, while the pH value doesn’t change that much.

So, by simply increasing the KH in your tank, you won’t necessarily make that much difference to your pH levels.

Q: Why Is KH Important to Your Aquarium?

A: KH measures the levels of bicarbonates and carbonates in your tank water, which can affect the water’s buffering capacity. That means KH effectively helps to neutralize acids, preventing your pH from fluctuating excessively.

That’s essential for the well-being of your fish since sudden pH changes can cause health issues for your livestock.

Q: What Is the Difference Between KH and GH?

A: Water hardness measures the concentration of magnesium and calcium in your aquarium water. Hardness is measured on a scale of 0 to 17, where the higher numbers indicate a higher level of magnesium ions and calcium.

There are two forms of water hardness: GH and KH.

KH relates to the levels of carbonate hardness in the water or the extent to which the water buffers pH fluctuations.

GH relates to the general hardness of the water, measuring the quantity of dissolved magnesium and calcium ions in the water. GH is essential for the healthy growth of certain fish species and plants.

Final Thoughts

I hope you enjoyed our guide on how to lower the KH in your fish tank. If you found the information helpful, please take a moment to share the article.

To maintain a stable level of water pH in your aquarium, the KH level needs to remain correct for the kind of fish and other livestock you keep. That’s essential for the well-being and health of your fish and plants.

You can adjust the KH in your water by using an acid buffer, adding Indian Almond leaves or Peat moss to your tank, or by using distilled or RO/DI water for weekly top-ups.

How did you solve the problem of low KH in your aquarium? Tell us how you did it in the comments box below!

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