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Self-Improvement & Hobbies

Water hardness

GH and KH – two water parameters you need to know besides pH
CATS Classified In The Straits Times - October 23, 2010
By: Wong Wei Chen
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Water hardness

I’ve gone on at length about pH, a measure of acidity and alkalinity in the aquarium. Maintaining pH, however, is not the only thing you’ll need to do.

While some species of fish are very hardy, and can survive in a variety of water conditions, the responsible aquarist will also need to look into other water parameters, so that fish can thrive and stay in the pink of health.

Since I’ve covered pH in detail, let’s look at two other important water parameters.

General hardness (GH)
GH, or “general hardness”, refers primarily to the concentration of magnesium and calcium ions in the water. Hard water has high concentrations of these ions, while the opposite is true of soft water.

General hardness can be measured either in terms of dH (degree of hardness) or ppm (parts-per-million). The table below gives you a rough overview of general water hardness:

Though generally not a critical parameter affecting fish survival, GH comes into play when you want to create conditions that will facilitate breeding among your fish, since this parameter can affect egg fertility and other reproductive factors. African and Central American cichlids, for example, need water of at least medium hardness to breed successfully.

Buffering capacity (KH)
Also known as buffering capacity or carbonate hardness, KH is mainly a measure of bicarbonate and carbonate ions in the water. It directly affects the water’s ability to retain a stable pH despite the introduction of acids or alkalis. A high KH entails stable pH and vice versa. Like GH, KH is also measured in terms of dH or ppm, and you can use the table for GH also as a gauge of carbonate hardness.

Drastic swings in pH spell trouble for fish. The buffering capacity’s ability to maintain a stable pH thus makes it a very important parameter. Your aquarium water’s KH should be high enough to prevent large pH swings over short periods of time. As a rough guide, if you find your pH changing up to half a point over a month, you should consider increasing the KH or doing partial water changes more frequently.

Changing water hardness
The overall hardness of water (not to be confused with either GH or KH), is a composite of both GH and KH. If you find yourself needing to change the water’s overall hardness, here are some ways you can do it:

Increasing hardness
To increase GH and KH simultaneously, you can add a small amount of calcium carbonate to your water. Approximately half a teaspoon per 100 litres of water will increase both GH and KH by 1 – 2dH. Alternatively, put some corals, seashells or limestone into your mechanical filter.

To raise KH without raising GH, you can opt for baking soda (sodium bicarbonate). Half a teaspoon per 100 litres raises the KH by about 1dH.

Decreasing hardness
Water softening can be easily achieved through dilution. Slowly dilute your tank water with distilled water, making measurements along the way, until the desired GH or KH level is reached. An alternative is to put peat into your filter and run your aquarium water through it. Bear in mind that water softening increases the chances of drastic pH swings, so always err on the side of caution. Soften your water a little at a time over an extended period, and keep close tabs on the pH level.


Shower time!