09-25-2010, 01:52 PM
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In the fish profiles it indicates the number of each species that should be kept together. Fish that need to be in a group of their own species are called shoaling fish; sometimes aquarists refer to them as schooling fish, but technically that is inaccurate due to the behaviour of true "schooling" fish compared to freshwater "shoaling" fish.
Shoaling fish must be kept in a group (usually 6 is considered minimum, sometimes more for behavioural reasons) in order to be healthy. Shoaling fish sometimes have a "pecking" order within the group, part of their natural instinct, and this interaction is a social behaviour that is essential for the fish's health. Shoaling fish kept alone or in 2 or even 3 can often develop aggressiveness beyond what is normal for that species, or normal aggression is made far worse. So to have healthy fish, attention must be paid to their numbers.
All of the characins (tetra, etc), rasbora, danio and barbs are shoaling fish. The minimum number of each species is six, sometimes more due to pronounced traits as in the Tiger Barb. In a 30g tank you are limited for the number of fish. As I mentioned previously, 8 Tiger Barbs only, plus the rainbow shark, would fill the 30g tank. If you do not have tiger barbs, then some of the other fish are possible, provided they have the number needed. Please read the fish profiles, all this info is there.