08-03-2011, 11:41 AM
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Tiger Barb is not a good community fish. You can read more in our profile, click on the shaded name. But quickly, they must be in larger groups (at least 8) and in their own space (30g minimum) or in a larger tank other fish can be present, but carefully selected as TB can be very nippy and aggressive.
The "odd vs even" idea has some merit when dealing with small groups, such as 3-4. And with some species. Some of the shoaling fish [shoaling means the fish live together naturally in large groups and thus need that security] can manage in 3-4, but the "pecking order" that Beaches mentioned is the issue here. A group of 3 could result in a pairing with an odd-fish out, and this can spell disaster with some fish. Having 4 or more alleviates this, though most shoaling fish are much better in groups of 6 or more. The male/female ratio mentioned by kitten is also pertinent. Some fish are better with more females to males. But aside from these two considerations, there is no evidence of even/odd differences. Then we come to personal preferences, also mentioned by Beaches. I myself always prefer odd numbers, just for aesthetics.
Back to your Tigers, no mention is made of the tank size, but you certainly should have at least 8, and then understand that this species can be trouble. It is in their nature, and some fish seem less "natural" than others when it comes to aggression.