I have two goldfish in an aquarium in my daughter's room. For the last couple of days, one of them looks really "fat" (for lack of a better word) towards the back fin and is having trouble staying low in the tank. It struggles to stay away from the water's surface. Also, it's having trouble getting it's head up to feed as if its backside is full of air and making it float up like a balloon. Does anyone know what the problem is? It is not acting sluggish or sick, but I'm worried that it's going to wear itself out trying not to float to the surface. The other fish is fine. Thanks for any help or suggestions.