Water hardness is a measure of the dissolved salts and minerals in the water, mainly calcium and magnesium salts. Water that is rich in these salts is considered 'hard', whilst water with relatively few salts and minerals is considered 'soft'.
Water hardness can be measured in a number of different ways but in fish-keeping it is usually measured as temporary, permanent, or total hardness.
Temporary hardness is a measure of calcium bicarbonate in the water and is considered temporary because it can be removed by biological processes and through boiling.
Permanent hardness is made up from calcium and magnesium sulphate and cannot easily be removed.
Total hardness, often called general hardness and measured in GH or dGH (degrees of General Hardness) is the combination of temporary and permanent hardness.
Temporary hardness, made up from bicarbonates is closely linked to pH, as it is these bicarbonates that provide a buffering effect and prevent pH from dropping. Because of this link, alkaline (high pH) water is often hard, whilst acidic water (low pH), is often soft.
Acidity/alkalinity (pH) and hardness should be treated as two separate water conditions however, as it is perfectly possible, and quite common is well established aquariums, to have water which is both hard and acidic.
In nature fish come from a variety of water conditions, some may originate in soft water whilst others may come from hard water, or may live in a wide range of hardness levels.
Water hardness is important when keeping fish that have evolved to live exclusively in hard or soft water, if kept at the wrong level of hardness these fish will suffer. Soft water fish for example, have evolved to cope with a relatively small amount of dissolved minerals; they do this by developing organs that are highly efficient at retaining minerals. If they are kept in very hard water, those highly developed organs may retain an excess of minerals, which may harden and cause premature organ failure.
Hard water fish kept in soft water have the opposite problem and they may not be able to retain enough minerals, having developed organs designed to expel excess minerals and not retain them. When this happens the fish may suffer from a lack of vital minerals that will affect many areas of the body, resulting in symptoms similar to malnutrition.