FRESHWATER FISH
These inhabit various biotopes in tropical and equatorial regions. Basically
they may be divided into two groups. The first comprises those for which soft, acidic
conditions are essential. Some require a very low level of hardness, with a typical
pH of around 6. The second group prefers hard, alkaline conditions. For some species
the level of hardness must be extremely high, with a pH of up to 8. A few species
can survive in brackish water. There are some fish which are not dependent on water
quality. These are consequently ideal for the beginner, who can, in principle, fill
the aquarium from the household supply. Though there are over 10,000 species native
to inland waters, only a few hundred need concern the aquarist.
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