What is KH and GH in Aquarium Water Test Kits?

In aquariums and fish farms, you need to be able to differentiate between Carbonate Hardness (KH) and Total Hardness (GH).

Carbonate Hardness

Carbonate hardness (KH or CH) refers to the ability of the water to maintain a stable environment and subsequently, the health of fish. The carbonate hardness is the ‘buffering’ ability of the water. To determine this on a photometer you need Palintest Alkalinity tablets, known as Alkaphot (part code AP188).

Total Hardness

General hardness (GH) refers to the ‘hardness’ of the water caused by calcium and magnesium ions. Different aquatic species prefer different environments, some prefer ‘hard’ water and some prefer ‘soft’ water. The hardness of your water is often dictated by the source of the water. If it’s from groundwater it tends to be harder, if it’s from surface water it tends to be softer. To determine this on a photometer you’ll need our Hardicol reagent (part code AP254).

Sometimes the units dH are used. This means “degrees hardness”. Most test kits define results in terms of ppm CaCO3 means ‘parts per million equivalent to that of calcium carbonate’ – it doesn’t mean that the hardness is in this form.

1 unit dH equals 17.8 ppm as CaCO3.

Aquarium Water Testing FAQs 

Below are some questions that we are frequently asked about aquarium water testing.

What is carbonate hardness (KH or CH), and how do I test for it?

Carbonate hardness is a measure of carbonates and bicarbonates dissolved in water. It refers to the ‘buffering’ ability of the water, which is how the water maintains a stable environment for aquatic life.

To determine carbonate hardness you need to measure total alkalinity. This can be done using a photometer and Palintest Alkalinity tablets, known as Alkaphot (AP188). Visual methods to measure total alkalinity are also available.

To view our range of photometers click here, and to view our visual testing products click here

What is total hardness (general hardness), and what reagents do I need to measure it?

Total hardness is a measure of the calcium and magnesium ions in the water, referred to as the ‘hardness’ of the water. Some aquatic species prefer ‘hard’ water, and some prefer ‘soft’ water.

Total hardness can be measured using our Hardicol reagent (AP254) on a Palintest photometer. This will give a result for total hardness in mg/L CaCO3.

Sometimes the unit’s dH are used for total hardness. To convert this result to dH divide by 17.8, as 1 unit dH equals 17.8 mg/L as CaCO3.


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