Water Hardness
Natural water contains a variety of dissolved substances which affect its characteristics, and this intern can have a drastic effect on the fish living in it.
Water hardness is a measure of the amount of dissolved lime and other minerals. Tap water from limestone areas is normally very hard: as it evaporates it leaves white lime deposits. Where water is drawn from granite areas it is very soft; that is, it has very little dissolved lime. Such water is sometimes amber-coloured, having filtered through peat beds. Each type of water has a place in the freshwater aquarium and, if necessary, the aquarist can make adjustments .Domestic water softeners should be used with caution, as they sometimes introduce other materials.
PH is a measure of the acidity or alkalinity of water. The neutral point on the pH scale is 7; waters registering below 7 are acid, while those above 7 are alkaline. Usually tap water has a pH between 6.5 and 7.5, and most fish can live happily under these conditions. A few specific types need very acid water, and these are described later under the relevant entries in the freshwater directory.
You can buy simple, cheap test kits which check the pH by means of dyes which change color under differing degrees of acidity or alkalinity. The pH can then be adjusted by adding solutions of sodium bicarbonate, to increase the pH, or of phosphoric acid, to decrease it (freshwater tanks only). This procedure is seldom necessary, however, unless one of the more sensitive types of fish is being bred.