Recently started a new tropical tank. Everything has been going fine, no problem with the fish- that is untill now! Three dead ones today. Have noticed that some of our fish have big belly's and initially thought they were pregnant. Now not so sure, although it is'nt the big belly fish that seem to be dying. Had noticed that one of the fish had started to look as if it was struggling to swim, and looked kind of paralised! Now having lost three fish in such a small space of time, I am worried for the rest of the fish! Would really love some advice! Does anyone recognise whats wrong with the fish, and can tell me what to do about it. HELP! 
