High KH is good for your tank. You’ve probably heard this since you’ve gotten your reef tank, but didn’t exactly get an explanation. The statement’s true, but only up to a certain extent. So, what does a high KH do and how do you lower it?

KH is the measure of acid neutralization, and it keeps the PH levels in your tank stable. The ideal range is 8-12 dKH. To lower KH, you can stop changing water, and use acid buffers, distilled water, RO/DI filtered water, Indian almond leaves, or peat moss.

KH is a term that you’ll hear bandied about often, without hearing a proper explanation about what it is. To add to the confusion, you’ll hear other terms being used interchangeably with it. Let’s take a deeper dive and explore everything you’ll need to know when it comes to KH.

What Is KH? What Does It Do?

KH is a popular way to refer to carbon hardness. It’s a measure of how much CO3 or carbonates and HCO3 or bicarbonates are dissolved in your reef tank water.

KH or carbon hardness is the key to keeping your reef tank water pH stable. pH stability is vital for your reef tank, as you’d encounter 1 of these 3 scenarios.

  • Fluctuating pH: Fluctuating pH levels can cause your fish to lose control over their hydration, becoming dehydrated or waterlogged.
  • Low pH: It creates an acidic environment where plankton and mosses thrive and compete with fish for resources. Acidity below 5 or 4.5 will eat away at your fish, with 4.5 being 10 times more harmful than 5.
  • High pH: High pH levels will affect the slime coating on your fish, causing chafing. It’ll cause erosion in their eyes, gills, and fins. Young fish will have their development stunted.

In all 3 cases, the situation will cripple and eventually kill your fish. KH stops this from ever happening, neutralizing the tank water acids and keeping the pH stable at the level you want. However, KH is also not a stable factor, and it degrades through neutralizing natural acidic buildup in the tank. It’s a good idea to keep an eye on your reef tank’s KH from time to time.

What Else Is KH Called? Is It The Same As GH?

The most appropriate name for KH is carbon hardness. It’s called KH because it’s not a contraction of the English name but the German name “Karbonatharte”. KH gets confusing because you’ll find many other terms being used interchangeably for it, including these.

  • Total alkalinity.
  • Alkalinity.
  • Buffering capacity/ buffer.
  • Temporary hardness.
  • Hardness.
  • Acid-neutralizing capacity or ANC.

Out of these popular terms, the term hardness can be a bit misleading and ties into the GH dilemma. GH is general hardness, indicating the levels of magnesium and calcium that are dissolved in your tank water. Hardness or hard water usually refers to GH, but someone can get confused and use these terms for KH.

How High Is Too High?

In general, keeping your KH high is considered good practice. It helps with the stability of the reef tank environment and makes it easier to maintain it. However, there’s also a range beyond which KH can cause issues for your fish.

The ideal KH range for reef tanks is 8-12 dKH. Deviating from this range doesn’t have the same result in both directions. Lower KH is more problematic than higher KH.

Any lower than 8 dKH, your alkalinity will degrade faster than usual, leaving your reef tank environment exposed to the effects of pH changes. It will also be harder to recover from pH fluctuations and lower pH.

If you have higher than 12 dKH alkalinity in your reef tank, it won’t have too bad of an impact. You can expect your fish to be fine if the level is raised to 14 dKH temporarily. However, sustained higher levels of KH can increase the pH levels in your tank, causing it to reach base levels. This will be incredibly harmful to your fish.

But in general, high KH is good for your fish. It won’t just keep the pH levels stable. It will deal with the nitrate and nitrite buildup in your tank. Nitrates and nitrites are the natural reasons for the lower pH levels in your tank, but they’re also toxic. High KH will cause chemical reactions that neutralize these compounds and remove the threat of toxicity.

How To Lower Your Reef Tank’s KH Level?

Decreasing your reef tank’s KH levels isn’t typically considered important. But there are some niche situations where you need to control lower KH in reef tanks. You’ll encounter these 2 issues more than most.

  • Having high pH and KH water, where you need to lower the pH.
  • Increasing the KH too much in your reef tank.

If you need to lower KH in your reef tanks, then here are some options you can try.

  • Wait And Do Nothing.
  • Distilled Water.
  • RO/DI Filtered Water.
  • Indian Almond Leaf.
  • Peat Moss.
  • Acid Buffer.

Wait And Do Nothing

When your reef tank KH is no higher than 14, you can wait it out. Your fish will be fine with a temporary increase in KH. The KH level will lower back to the acceptable range with nitrate and nitrite buildup from their excrement. However, this isn’t ideal when you’re dealing with a high pH issue in your reef tank.

Distilled Water

Distilled water is pure water that’s been turned to steam and then cooled back to liquid form. It loses all carbonates and bicarbonates in the process, having no KH at all. It’s also at an ideal pH of 7, which is neutral. You can mix this in a 1:1 ratio with tap water to create an ideal pH and KH mix for your reef tank.

However, there are 2 downsides to Distilled water. It can come into contact with carbon dioxide in the atmosphere and change its pH to an acidic 5.8. It can cost $6-$20 per gallon, which can get very expensive considering how much water you need for your reef tank.

RO/DI Filtered Water

You can also get water with no KH using either reverse osmosis or RO and deionization or DI filter. It behaves similarly to distilled water, and you have to mix in tap water to introduce KH proportionally into your reef tank water.

The issue with this option is that both RO and DI filters will initially cost $550-$2000 to set up. But you can get rid of the ongoing cost of using distilled water. Also, you can expect a perfect pH 7 from DI filters if you hook it up directly to your reef tank. For reef tanks, this setup is typically considered ideal.

Indian Almond Leaf

A gentle solution to lowering your reef tank KH is to use Indian almond leaves. These release tannic acid as they break down, which is neutralized by the KH in your tank. As a result, your reef tank KH lowers faster.

However, this change isn’t fast. It beats waiting for a lower KH, but it won’t make a drastic change rapidly. This method is meant for subtly lowering your KH, chipping away at it than knocking down entire levels. It’s a reliable method, as it won’t shock your fish with fluctuations.

Peat Moss

Peat Moss is another gentle method to lower KH with some degree of accuracy. It also releases tannic acid into your reef tank water and gradually eats away at the KH. It’s a subtle method with a greater degree of accuracy than the Indian almond leaf.

To use peat moss, put it in a mesh bag and place it in your reef tank. There are many unsafe variants of peat moss that you should avoid, so check if the peat moss is labeled aquarium safe.

Acid Buffer

Acid buffers are more aggressive solutions to lowering your KH and pH levels. An acid buffer converts the carbonates and bicarbonates in your tank to carbon dioxide. Carbon dioxide is naturally acidic, and it rapidly decreases the reef tank’s KH.

Acid buffers are typically used in tanks with plants that can remove extra carbon dioxide. Plants in reef tanks are uncommon, so you won’t have anything to compensate for overusing acid buffers. If you use acid buffers, you risk the sudden possibility of an acidic tank and your fish dying.

To use an acid buffer, use 2 grams for every 80 liters. This will lower the alkalinity by 0.6 dKH but use a lower concentration. As your reef tank has little protection against overdosing, go as slow as possible to avoid any unfortunate accidents.


Reef tanks have a high tolerance for KH, and typically you shouldn’t need to worry about it. But if you need to make changes, then wait or use Indian almond leaves. For rapid changes, use distilled water or a DI setup. Only use acid buffer when you have no other choice.

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