when I got my purple firefish, I had the same questions. I was told that it is only a good idea to keep two if you know for sure they are a mated pair. I don't know if that is right, but I just kept the one. I also think it is better if you add two at the same time, and could run into problems if one is already in the tank and then you add another. I don't know if this is correct, because so many places give varying opinions. Anyways mine is a loner, and he is just great, never hides, swims the whole tank, I think I got lucky because many people say they hide all the time, but mine is fearless!!