Hi, this is my friends goldfish, she has a 45x25x30 fish tank with 4 small goldfish (used to be 5, the 5th one died because of the same reasons) we don't know what might be wrong with him, or what is that is causing this. I don't know how to help her as i do not own any cold water fish. Please help !! (sorry for bad grammar english is not my first lenguage).