GH & KH Explained

The difference

GH and KH are not the same thing.

General hardness

GH tells you about the mineral content of the water.

General hardness mainly reflects the amount of dissolved calcium and magnesium in freshwater. These minerals play important roles in the health of aquarium animals and plants.

Water containing relatively little calcium and magnesium is described as soft. Water containing more of these minerals is described as hard.

Neither soft nor hard water is automatically better. What matters is whether the water suits the species you are keeping.

Why GH matters

Different species are adapted to different mineral levels.

Some freshwater fish naturally occur in very soft water, while others are better suited to harder, mineral-rich conditions. Keeping animals far outside their preferred conditions can cause problems over time.

GH is particularly important when keeping freshwater shrimp and snails because calcium and magnesium are involved in normal moulting, shell development and other biological processes.

Carbonate hardness

KH helps keep pH stable.

KH describes the carbonate and bicarbonate buffering capacity of the water. In simple terms, it helps the aquarium resist sudden changes in pH.

Acids are naturally produced in an aquarium through biological processes. Carbonate buffering can neutralise some of that acidity, helping prevent the pH from changing too quickly.

GH, KH & pH

The three measurements are connected, but they are not interchangeable.

It is easy to assume that hard water must have a high pH or that soft water must be acidic. Real aquarium water is more complicated than that.

GH tells you mainly about calcium and magnesium. KH tells you about carbonate buffering. pH tells you how acidic or alkaline the water is at that moment.

Because KH contributes to buffering, it has an especially important relationship with pH stability.

Measuring hardness

Check the units before comparing results.

Aquarium hardness is commonly reported as degrees of general hardness and carbonate hardness, written as dGH and dKH. Water reports and some tests may instead use parts per million.

One degree of German hardness is approximately 17.9 ppm when expressed as calcium carbonate equivalent.

This means two numbers can look completely different while describing roughly the same hardness, simply because different units are being used.

Testing your water

Start with the water coming out of the tap.

Testing both your source water and aquarium water gives you much more useful information than testing the aquarium alone.

If the readings are noticeably different, something inside the aquarium may be affecting the mineral content or buffering capacity. Rocks, substrates and remineralising products are common examples.

• Follow the instructions supplied with your test kit.
• Count drops carefully when using liquid titration tests.
• Write down results so you can compare them over time.
• Retest an unexpected result before making major changes.

Neocaridina shrimp

Mineral balance matters when shrimp moult.

Neocaridina shrimp generally do well in moderately mineralised freshwater, but GH should never be considered on its own. Temperature, KH, pH and overall stability all matter as well.

Calcium and magnesium are important during normal moulting and exoskeleton development. Water that is extremely soft or poorly balanced can therefore cause problems even when the pH appears suitable.

Snails

Shell-building animals also depend on suitable minerals.

Calcium is particularly important for shell growth and maintenance. Very soft or acidic conditions can make shell erosion more likely in some commonly kept freshwater snails.

This does not mean every snail should automatically be kept in very hard water. Always consider the requirements of the particular species you are keeping.

Aquarium plants

Calcium and magnesium are plant nutrients too.

Aquarium plants require both calcium and magnesium for healthy growth. In many aquariums these minerals are already supplied by the source water.

Very soft water or reverse-osmosis water may contain much less, which is one reason remineralisation can be important.

GH is only one part of plant nutrition, however. Nitrogen, phosphorus, potassium, trace elements, lighting and carbon availability also influence growth.

Reverse-osmosis water

RO water gives you control, but usually needs minerals added back.

Reverse-osmosis filtration removes a large proportion of dissolved minerals and other substances from source water.

This can be useful when the local tap water is unsuitable or when an aquarist wants greater control over the final water chemistry. However, very low-mineral RO water should not simply be treated as ideal aquarium water.

It is commonly remineralised before use so the final GH and, where appropriate, KH suit the livestock being kept.

Raising GH

Add minerals deliberately, not by guesswork.

GH can be increased using suitable remineralising products that supply calcium, magnesium or both.

Some rocks and substrates can also release minerals into the water. This may be useful in certain setups, but it can make precise control more difficult if you do not know how strongly the material affects your water.

• Test before adding anything.
• Know what the product contains.
• Prepare replacement water consistently.
• Make changes gradually.

Raising KH

Increasing KH also changes the aquarium's buffering.

Carbonate- and bicarbonate-based materials can increase KH. Calcareous rocks and substrates may also increase carbonate hardness as they dissolve.

Because KH is closely connected with pH stability, changing it can affect more than one water parameter at the same time.

Lowering hardness

Sometimes the easiest solution is to start with softer water.

If your source water is significantly harder than the animals you intend to keep require, it can be mixed with lower-mineral water such as reverse-osmosis water.

The final mixture should be tested rather than assumed. Once you find a suitable mixture, consistency between water changes becomes important.

Evaporation

Water evaporates. Most dissolved minerals stay behind.

When water leaves the aquarium through evaporation, the minerals dissolved in it generally remain in the tank.

If evaporation is repeatedly replaced with hard tap water, more minerals are being added each time and hardness can gradually increase.

This is different from a water change. During a water change, some aquarium water and its dissolved minerals are removed before fresh water is added.

TDS

TDS is useful, but it does not replace GH and KH testing.

TDS stands for total dissolved solids. A TDS meter gives a broad indication of how much dissolved material is present in the water, but it does not tell you what that material actually is.

Two aquariums can have similar TDS readings while having very different GH, KH and overall water chemistry.

This makes TDS useful for spotting changes and maintaining consistency, especially when preparing remineralised water, but it should not be treated as a replacement for proper hardness tests.

Common mistakes

Most hardness problems come from changing things without understanding the water first.

• Treating GH and KH as though they measure the same thing.
• Assuming pH tells you whether the water is soft or hard.
• Adding minerals without testing the source water first.
• Using very low-mineral RO water without appropriate remineralisation.
• Making large hardness changes while livestock are still in the aquarium.
• Using TDS as though it identifies individual minerals.
• Trying to hit one exact number instead of maintaining a suitable and stable range.

A sensible approach

Learn what your water is already doing before trying to change it.

Start by testing the GH, KH and pH of your source water. Then test the aquarium and compare the results.

If the aquarium readings are different, look at what could be responsible. Rocks, substrate, remineralisers and other materials can all influence water chemistry.

Next, compare those conditions with the actual requirements of the animals you keep. A number should only be changed when there is a good reason to change it.

Quick recap

GH = minerals. KH = buffering.

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