What Is kH in Aquarium? How to Stabilize it?

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Monitoring physical and chemical parameters is part of tank maintenance. Aquarium geeks, like me, tend to do this everyday as we would like to give the best environment for our fish.

Yesterday, when I measured the acidity and alkalinity of my tank water by reading the pH level, I noticed that it slightly went up. Fortunately, the increase is minimal and poses no significant threat to my fish. However, today it significantly decreased and I am both alarmed and confused.

What is happening with my pH? I asked fellow aquarium geeks and they told me it has something to do with kH.

But what is kH? Is it different from pH?

Join us in this article as we discover what kH is and how its associated ions affect the pH of water in our tank.

What is kH?

Also known as carbonate hardness, kH refers to the buffering capability of water that is influenced by the quantity of carbonates and bicarbonates. These ions are negatively charged and will react with acid by breaking them down.

Its unit of measurement is expressed in degrees of kH (dKH) or parts per million (ppm). The equivalent of 1 dKH is 17.86 ppm.

When it comes to the relationship of kH and pH, it is directly proportional. This means that if the kH level decreases, the pH level will also decrease and your water becomes acidic. This is due to the decreased buffering ability of your water against acids.

On the other hand, if the kH level increases, the pH level will also increase making your water alkaline.

When the level of carbonates and bicarbonates present in the water are stable, then it would also lead to a stable pH and the possibility of a fluctuation (like what happened in my tank) will not occur.

kH: Vacuum Cleaners of Acid

kH, as well as pH, is not visible to the naked. It is best explained if we place a representation and kH is best represented as a vacuum cleaner for acids.

The majority of the acids formed in the aquarium come from the waste product of our fish. The most common form of acid formation in the tank is when fish waste releases ammonia, which is broken down and degraded into nitrites. As these nitrites accumulate, they will be converted into nitric acid.

With kH, acids cannot accumulate as they are constantly broken down by the negatively charged ions converting them into simpler compounds.

In other words, kH vacuums acid out of the water. The higher the kH level, the more it vacuums out acid. On the other hand, the lower the kH level, the less it cleans acid.

Manage kH First to Manage pH

Commonly, we focus on managing the pH level and often disregard kH. I am guilty of this.

In my early days as an aquarist, I sometimes wonder why my pH level did not change despite the measures I made. I often add soda ash whenever the pH of my water drops and becomes acidic. Inversely, if the pH increases and my water becomes alkaline, I apply sodium hydroxide.

Despite introducing these substances to counter pH, there was no reversal and neutralization did not occur.

After understanding the relationship between kH and pH, I focused on managing kH first. Voila and to my surprise, without the use of any pH inducing substances, I did manage my pH to a level that I wanted.

Later on, you will learn how to manage and stabilize your kH.

Related: High Aquarium pH: Causes, Symptoms, & Cures

Ideal kH Level in Different Tanks

There is no standard level of kH. You should be aware that different species of fish and other live tank inhabitants require a range of pH levels. Thus, with its direct influence on pH, kH varies accordingly.

Listed here are the common kH ranges in different tank set-ups.

Type of Tank kH Level
dKH ppm
• Freshwater 4 – 8 71 – 143
– Shrimp Tank 2 – 5 36 – 89
– Planted Tank 3 – 8 54 – 143
– Cichlid Tank 10 – 18 179 – 321
• Brackishwater 10 – 18 179 – 321
• Saltwater 8 – 12 143 – 214

In general, the kH level in freshwater tanks should be anywhere from 4 – 8 dKH. However, there are cases that kH levels need to be either decreased or increased to accommodate certain species of fish and other live inhabitants.

In the case of freshwater shrimps, it needs a low kH level. This is ironic since they need carbonates (in the form of calcium carbonate) during molting. Shrimps only need a small amount of carbonate and that high kH water can cause their exoskeleton to thicken and be difficult to break during molting.

Similarly for live plants, they need a low kH level.

When deprived of their primary energy source, plants use carbonates as an alternative source of carbon. In fact, plants grow fast at high kH levels. However, it will be exhausting to the plants since converting carbonates to carbon requires a lot of energy.

It will be a different setting when it comes to cichlids. Since they mostly flourish in alkaline waters, their kH requirement is also high.

Fun fact: Pure distilled water has zero kH. When a small amount of acid is introduced, it will cause rapid pH change. This is the reason why you should never use pure distilled water in your tank.

Test Strips: An Essential Tool in Measuring kH Level

Checking the kH level is often done with measuring other variables. The reason for this is most test kits are multi-parameters. Test strips are the most common forms of checking the kH level.

Test strips are very easy to use. Depending on the manufacturer’s instructions, dip the test strip onto the tank water for a few seconds.

The procedure in using test strips is like doing a swab test. However, this is not done on the nose like for covid, but in the water for kH.

After dipping, take it out and hold it still for a few minutes. You will notice that several sections of the test strip will change in color. From white, it will turn to one of the many colors in the rainbow spectrum.

Each color section corresponds to a parameter. Look for the kH parameter in the test strip and match its color provided in the bottle or in the printed guide. Each color represents a specific value.

Again, depending on the manufacturer’s instructions, a yellow color on the kH section of the test strip is an indication of 0 dKH or 0 ppm. It means that it has no carbonates and bicarbonates, just like pure distilled water.

As the color progresses into a darker shade, the kH value also increases. Green may indicate a kH level of 16 dKH or 300 ppm.

The Effects of Low kH

Although indirectly, it is the fish that suffers the most in a low kH event. Indirect in the sense that the low kH level will first influence the lowering of the pH, thus creating acidic waters which fish are highly sensitive to.

It is given that, in a fish tank setup, acids from their waste are continuously generated. If the kH level is low, then it will allow the accumulation of acid. In effect, acidic water is produced.

2 dKH is critically low. Even the slightest production of acid from waste can destabilize your pH.

Moreover, low kH water is the perfect environment for algae to flourish. They love acidic waters. This is the reason why algae blooms in large numbers when you have too many fish in the tank with a poor filtration system.

Related: How to Get Rid of Algae in the Fish Tank?

Carbonate-Rich Additives that Increase kH

Carbonate-Rich Additives that Increase kH
Credit: @the_fish_crew

To address low kH, adding carbonates can significantly increase its level. In nature, there are a lot of carbonate-rich substances that we can use to counter the effects of acidic water.

Sodium Bicarbonate in Baking Soda

Also known as sodium bicarbonate (NaHCO3), baking soda is an effective additive that increases kH.

But make no mistake in just adding any amount of baking soda to your tank water. You might spike up the kH so much that the water exceeds neutrality and becomes alkaline.

The recommended formulation is 1 teaspoon of baking soda for every 50 liters (13 gallons) of water. If you get the correct ratio for the amount of baking soda versus the total volume of water in your tank, then you can expect equilibrium to happen and levels will be maintained at 8 dKH.

Calcium Carbonate in Crushed Coral and Aragonite

Derived from dead sea corals, crushed corals are effective in boosting up your kH level. Corals contain high amounts of calcium carbonate (CaCO3).

Due to their carbonate content, adding them in the water will almost instantly spike up the kH level.

Similarly, aragonites are the crystal form of calcium carbonate and they look like tiny sand-like particles.

Ideally, both crushed coral and aragonite can either be placed directly in the bed (together with the sand) or wrapped in a cloth bag. While they require little or no maintenance at all, you need to replace them after 6 months so that mineralization in your water is continued and sustained.

Calcium Magnesium Carbonate in Dolomite

Classified as a calcareous rock, dolomite is mainly composed of CaMg(CO3)2 or calcium magnesium carbonate.

When placed in acidic water with low kH level, dolomite will immediately react and release carbonates. While they are effective in freshwater tanks, they do not perform well in saltwater aquariums.

Fun fact: Dolomite comes in different colors (like white, pink and blue). Pick your favorite color or match them with your sand color as they are highly recommended to be placed in the tank bed.

Sodium Carbonate in Soda Ash

Especially if you are into saltwater aquariums, soda ash will greatly aid kH level increase in your tank.

Also known as sodium carbonate, the addition of soda ash is ideally done in small quantities. This allows you to control the rise of kH without compromising sudden pH change.

Commercial Alkaline Buffer

Most of the carbonate-rich substances we presented above are naturally occurring in our surroundings. Sometimes, finding them takes time and great effort.

With online shopping at the comforts of your home, you can purchase commercially made alkaline buffers. However, these products are mostly derived from the natural substances we just presented.

The Effects of High kH on Fish

Not unless it’s on the extreme (like 20 dKH or more), then nothing should happen to your fish.

Take note that there are certain fish species (like cichlids) that love alkaline water with high kH levels. If you have this kind of set-up, make sure it is a species-only tank and do not mix other fish. Most fish thrive in neutral water and exposing them to alkaline can cause mortality.

Natural Supplements that Decrease kH

Like the use of raw materials in increasing kH, there are natural substances that can also decrease kH levels.

Tannic Acid in Peat Moss and Indian Almond Leaf

Peat moss and Indian almond leaf are excellent sources of tannic acid. They are usually placed in a cloth or mesh bag for immersion in the water.

As the peat moss and Indian almond leaf degrade in the water, they release tannic acid. These emitted acids will put the carbonate to work, thereby decreasing the kH level.

Distilled water

As a by-product of boiling, distilled water contains no minerals. Therefore it has zero kH. Moreover, it is slightly acidic in pH and contains carbonic acid.

Commercial Acid Buffer

Looking for peat moss, Indian almond leaf or making distilled water is costly and laborious. But no worries. Commercial acid buffers can easily be purchased at online platforms.

Most of the commercial acid buffers are aimed at planted tanks. These chemicals extract the excess carbon dioxide produced by plants. So, be careful not to place too much acid buffer as you might end up killing your fish.

Water Change: A Natural Remedy for Extreme kH

In case all the above-mentioned substances are not available, changing the water is not just your last resort, but it is also a natural remedy for stabilizing kH.

Whether you have a low or high kH level, changing your old tank water with a fresh one will neutralize your kH.

You don’t need to totally change the entire water volume in your tank. The amount of water change will depend on how low or high your kH is and how far you want it to increase or decrease.

It is best that you check your kH level first before doing any water change. In general, changing 25 – 30% of the water will stabilize the kH.


Of all the ways to manage kH level, it is water change that is most tedious and tiring. But why wait to step into your last resort when you can easily manage kH with a few additives?

But before you can manage, you need to know your current kH level. With this, it is highly recommended that regular kH testing should become your routine. In this way, when the readings tell you it’s getting off the charts, then you can easily do preventive maintenance.

In my case, I was fortunate enough to apply baking soda and reverse my acidic water, as coincidentally that day, I was making pancakes.

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