Can different species of rainbow mix?
I assume you are thinking of adding different rainbows to make up the numbers, so the answer is no. But as for different species of rainbowfish getting along, generally yes. In a larger tank you could have more than one species of rainbow, though you have to watch the water parameters as some are soft water and some moderately hard water fish.
The issue with numbers of a species, what we term shoaling fish (some call these schooling fish, a bit different technical meaning but the same principle), is primarily one of security. These fish have evolved to live in large numbers, and they expect this. Some species have hierarchies within the group. Others may have territorial/aggressive issues that need more of the species to avoid some getting bullied and picked on. Studies have now proven that when shoaling fish are in small numbers, below six, their aggressiveness increases. This is undoubtedly the fish's response to a very frustrating situation. The fish in the aquarium cannot move away from problems like it might in the natural habitat, so it either accepts things or it fights back. Whichever response the fish chooses, it adds stress which weakens the immune system and the fish will usually succumb to various health problems that it would ordinarily be able to fight off if not stressed by the insufficient numbers. Small physical space, as in too small a tank, also causes the same reaction in fish, which should not be surprising. So, the bottom line is, that if one want their fish to be "happy," normal and healthy, and exhibit natural behaviours, one must understand what is required and provide it.
Byron.
