I have these two goldfish in a tank together. The tank is quite big enough for the two of them, and one of them seems to be completely fine. However, the other one seems to have trouble staying down in the water; that is, if he's not swimming down he will float to the top. This first started several months ago. Sometimes I will look in on the fishies and he'll be perfectly fine, and other times he'll be floating on his side and I'm sure he's dead, only when I come closer he sees me and swims to the bottom. I keep the tank clean and everything and they're both quite healthy looking; however I don't know much about fish other than they need to be clean and fed. Does anyone know why this might be? Does he have some kind of swimming deficiency or something?