General Hardness
General hardness (GH) is primarily the measure of calcium (Ca++) and magnesium (Mg++) ions in the water. Other ions can contribute to GH, but their effects are usually insignificant, and the other ions are difficult to measure. GH will not directly affect pH although “hard” water is generally alkaline.
When a fish or plant is said to prefer “hard” or “soft” water, this is referring to GH. Incorrect GH will affect the transfer of nutrients and waste products through cell membranes and can affect egg fertility, proper functioning of internal organs such as kidneys, and growth. Within reason, most fish and plants can successfully adapt to local GH conditions, although breeding may be impaired.
Carbonate Hardness
Carbonate hardness (KH) is the measure of bicarbonate (HCO3-) and carbonate (CO3–) ions in the water. In freshwater aquariums with neutral pH, bicarbonate ions predominate. Alkalinity is the measure of the total acid binding capacity (all the anions which can bind with free H+) but is comprised mostly of carbonate hardness in freshwater systems. Thus, in practical freshwater usage, the terms carbonate hardness, acid binding, acid buffering capacity and alkalinity are used interchangeably.
When the aquarium has some carbonate buffering in it, the bicarbonate ions will combine with the excess hydrogen ions to form carbonic acid (H2CO3), which slowly breaks down into CO2 and water. Since the excess hydrogen ions are used in the reaction, the pH does not change very much. Over time, as the carbonate ions are used up, the buffering capacity will drop, and larger pH changes will be noticed.
Adjusting Hardness
If your local water is too hard for the fish and plants you desire, it can be softened. There are many ways to do this, but some are more suited to aquarium use than others. The best and most expensive is to use is a reverse osmosis (RO) deionizer and mix the resulting water (GH=0) with tap water, to get the desired GH.
Commercial water softening resin “pillows” can be used on a small scale (aquariums) but are not effective for larger amounts of water. Water softening systems designed for large scale home use (like bath water) are not suitable, since they use an ion exchange principle, whereby usually sodium ions are substituted for calcium and magnesium ions, and excess sodium is not desired in ponds or aquariums. An even worse practice is to use a cation exchange resin in the hydrogen ion form and use it to pull divalent ions out of the water.
If the local GH is too low, it can be raised by adding calcium sulfate and/or magnesium sulfate. This has the drawback of introducing sulfates (SO4–) into the water, so care should be exercised. Calcium carbonate can be used, but it will also raise the KH. This is ideal for the lucky few who have naturally soft water. Various combinations can be used to produce the desired results.
Carbonate hardness is too high it can be reduced by boiling the water. However, this is impractical for anything other than small aquariums. Carbonate hardness can be increased by adding sodium bicarbonate. Calcium carbonate will increase both KH and GH in equal parts.
