In the introduction to water chemistry, I mentioned how fish have evolved to adapt to their natural surroundings and for a fish to be happy and healthy in an aquarium, the water's chemical composition must be similar to that of the their natural habitat. But what is their natural habitat like?

This section provides an overview of water conditions from several regions around the world. It does not take into account specific local geological differences nor seasonal fluctuations in mineral content and pH. None the less it will act as a guide to help the hobbyist create the perfect habitat.

SOUTH AMERICA:

The Amazon region is home to the majority of aquarium fish and the natural habitat of tetras, angelfish, discus and dwarf cichlids. Three types of water occur in this area:
White water, is strangely very murky. It is found in the main branch of the river and contains a high clay content making it moderately soft (50-100 ppm) and acidic (pH 6.5 - 7.0). The abundance of plant growth in this area helps to keep nitrates at trace levels.
Clear water, as its name suggests is transparent but slightly yellow to dark green in colour. It is very soft (<50ppm) and more acidic (pH 4.5 - 6.5) than white water. Again there are only traces of nitrate.
Black water, found in the Rio Negro and surrounding areas, is dark brown. The water is even softer and more acidic than clear water (pH 3.8 - 5.5).

CENTRAL AMERICA:

Home to live-bearing tooth carps, the water is medium hard to hard and slightly alkaline. Some of these fish are also found in brackish water which contains a low salt content.

NORTH AMERICA:

Perches thrive here. They prefer slightly cooler water than most tropical aqaurium fish (around 72°F) and are best kept in hard water (300-450 ppm) with a neutral pH.

SOUTH EAST ASIA:

Danios, loaches and gouramis are found in practically neutral water but very soft (<50 ppm). The Malayan Archipelago is the largest source of aquarium fish outside South America. Harlequin fish and many barbs live here. The water is very clean and contains virtually no minerals. It has a pH of 6.0 and a hardness of <30 ppm.

AFRICA:

Two areas are of interest here. The East African Lakes, primarily Lake Malawi and Lake Tanganyika and the Western and Central African rivers.

The Lakes are the primarily inhabited by Cichlids and contain very hard alkaline water (>300 ppm, pH around 8.5). By contrast, the Stanley Pool of Zaire, home to the egg laying cyprinodonts, carps, has soft water (<50 ppm) and an acidic pH of 6.5.

EUROPE:

The few European fish suitable for the aquarium are very adaptable and can be kept in water varying from 100-450 ppm and neutral pH.