As an aquarist, pH, GH, and KH are words you constantly use. You understand how essential stable water parameters are for a healthy aquarium. Correct water chemistry helps preserve your fish’s health and reduces the mortality rate.

Knowing your water’s chemical characteristics, such as the hardness, acidity levels, and so on, will help your fish remain in optimal health. If the environment in your aquarium is not suitable, your fish will be stressed and more likely to fall ill.

Although pH, GH, and KH are terms commonly associated with water chemistry, they sound a little intimidating. This guide explains the differences between pH, KH, and GH and their impact on your fish.

What Is pH in an Aquarium?

pH stands for Power Of Hydrogen. pH gauges the hydrogen ions in your fish tank. It shows the acidity or alkaline, i.e., basic level of the water. The measurement of pH levels is on a scale from 0 – 14. A lower pH shows more acidic water, while a higher pH means the water is more alkaline. Water that is pH 7 is neutral.

– What Is the Best pH Level for Aquariums?

Not all fish tolerate the same pH levels. The level that suits one breed may not work for another species. Thus, it is critical to research before you add any new fish to your community tank. You need to know what pH levels your fish require.

Generally, most freshwater breeds are happy with a relatively neutral level of 6.5 to 8.0 pH. However, some fish prefer a lower pH, like those native to South America or the Caridina crystal shrimp. On the other hand, African cichlids and livebearers enjoy a higher pH.

Small pH fluctuations in planted aquariums are a natural result of oxygen and carbon dioxide production during photosynthesis. Generally, these are minimal and will not pose a problem for your fish since they are used to similar changes in their natural habitats.

The main thing is to maintain a relatively stable pH. Most fish will adapt to a change in the pH if it is not a sudden spike.

– What Impact Does pH Have?

The pH of your tank water affects water quality and water chemistry. Waste products from fish such as excrements and decomposing food particles may raise pH. So, if the pH drops below 6.0, the bacteria maintaining ammonia and nitrites at zero ppm will die. The number of toxic compounds in the water will increase, causing fish to die.

What Is KH in Your Tank?

The KH or carbonate hardness measures the bicarbonate and carbonate ions in your water.

KH is the buffering capacity or the ability of water to keep the pH stable. Carbonate ions bond with hydrogen ions, the pH.

Consequently, when more carbonate ions bond, the pH level increases, while fewer carbonate ions decrease the pH level in the tank. The KH is vital because it determines the stability of the pH level in your aquarium, directly impacting the health of your fish.

When KH neutralizes the acids present in the tank water, it prevents the pH from changing swiftly and crashing. Avoiding crashes is crucial since it can harm and potentially kill your fish.

A Low KH indicates your water has an inadequate buffering capacity, so the pH level swings quickly. On the other hand, if the KH is high, your water has a higher buffering capacity, making it difficult to change the pH level.

– What Is the Ideal KH Level for Aquariums?

KH is measured in dKH, i.e., degrees of KH or ppm, i.e., parts per million, where 1 dKH equals 17.9 ppm. Freshwater aquariums should typically be between 4 – 8 dKH or 70-140 ppm.

If it is necessary to lower the KH for marine animals such as Discus fish or Crystal shrimp, you’ll need to reduce the KH to 0 – 3 dKH or 0 – 50 ppm. On the other hand, African cichlids prefer a KH higher than 10 dKH or 180 ppm, compatible with higher pH levels. Therefore, it is vital to check the requirements for each species before introducing them to a community tank.

– How to Raise or Lower the pH

Fish can adapt to various pH levels, provided it’s not too far out of the ideal range. However, fish can not tolerate a constantly changing pH range and may die.

It is best to change pH gradually to allow your fish to adapt to a new level.

A standard practice to increase the aquarium’s pH is to add baking soda. It’s wise to remove the fish from the tank before changing the pH. The correct way to add baking soda is first to dissolve the required amount in a pail.

You can mix dolomite chippings or crushed coral in your substrate. These gravels contain calcium carbonate that gradually dissolves into the water.

You should remove 20 percent of the water in your tank every two or three weeks. Small, regular changes are safer than changing large amounts that could shock your fish.

A safe way of decreasing pH without changing the KH is to aerate the tank with Carbon Dioxide or CO2. When the CO2 dissolves, some of it forms carbonic acid, which serves to lower the pH.

What Is GH in Your Tank?

GH or general hardness tells you the amount of calcium and magnesium ions present in the water. It is a measurement of the hardness or softness of your tank water. It is advisable to have a GH level between 4 – 8 dGH.

Suppose you would like your fish to thrive. You must ensure your aquarium water has sufficient salts and minerals essential for healthy biological functions, such as bone and muscle development of fish, snail shell development, shrimp molting, and plant growth.

– What Is the Ideal GH Level?

GH is also measured in dGH or degrees of GH and ppm. The optimal level for freshwater aquariums is a GH level between 4 – 8 dGH or 70 – 140 ppm.

While all animals need minerals, species such as livebearers, goldfish, and African cichlids are happier with higher GH levels. However, if you’re trying to breed discus or other soft water fish, reducing the GH to 3 dGH or 50 ppm or less may be advisable.

If your tank has low GH, you may notice that your fish isn’t eating well. They may be lethargic, or their colors may begin to fade. If you have shrimp in your tank, they may have trouble with molting, or snails will have thin, pitted, or flaking shells.

Low GH also affects plants in your aquarium. They may be dying due to calcium or mineral, and electrolyte deficiencies.

It is essential to remember that GH measures both magnesium and calcium levels. If the GH in your water is high, but your plants and animals are not thriving, the tank water may have ample magnesium but insufficient calcium.

It is best not to let the GH values in your aquarium get below the required level because it may lead to slow growth or even the death of your animals and plants.

– How Are pH, KH, and GH Related?

Although pH, KH, and GH measure specific ions, they are interrelated and balance the water chemistry. When you add natural minerals to your tank, they release numerous ions that affect various water parameters.

Limestone, for instance, contains a high level of calcium carbonate. Since it includes both calcium and carbonate ions, you raise both GH and KH when you add them. African cichlid owners often purchase salt mixes specifically created to raise either KH or GH.

KH is directly connected to pH. It stabilizes the pH, preventing it from changing quickly. Since pH levels tend to decrease in aquariums with time, raising the KH neutralizes more acid, and pH stays higher.

For instance, adding a buffering agent like crushed coral to a higher pH level will only raise the KH, not the pH value. However, adding crushed coral to a lower pH will increase both pH and KH values.

If the KH in your tank drops, you need to increase the carbonates and bicarbonates in the water. You can get commercially prepared alkalinity buffers when you need to raise the KH in your freshwater aquariums. They are an excellent aid and make the process easy.

Crushed coral mixed with aragonite is an excellent alkalinity buffer. The benefit is that it changes KH slowly and steadily.


As a fish enthusiast, you try to ensure the health and safety of your fish. One of your primary concerns is water chemistry and regulating the pH, KH, and GH levels. Let’s summarize this information.

  • pH stands for Power Of Hydrogen.
  • It is essential to maintain a stable pH.
  • The pH of your tank water affects water quality.
  • KH is the buffering capacity of water.
  • GH is the hardness or softness of the water.

We hope this information assists you with the water quality in your tank, so your fish remain healthy and give you many hours of pleasure.

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