12-05-2009, 07:36 PM
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What Angel is mentioning is a bit deeper than what you can see in the tank. Tetras are characins, and basically all species of characin are shoaling (schooling is sometimes used) fish. This means they remain together as a group in the wild. The majority do not hunt or feed as marine schooling fish do, but they do prefer to be together. When they are in a group of their own species, they are less stressed and therefore more healthy and less likely to develop any number of problems associated with stress. Not much different from humans actually
On their own, they may "appear" OK, but they perceive themselves as vulnerable. Their natural instinct tells them that they should be in a group of their own species to be "safe," but they cannot find any in the aquarium. This is bound to be stressful. Kept in a group, they may be expected to not only be healthier but live longer as a result.
I would agree with Angel that you should decide which ones you really like and would like to have in a group of 5 (that's about the minimum) and perhaps the local fish store will do an exchange.