My daughter's fish (which I'm taking care of while she's in college) seems to be sick and won't eat. I have cleaned the tank and replaced the filter but the fish hasn't eaten in at least 5 days. He was hanging around on the bottom for about a week but after I cleaned the tank he started swimming around again. He holds his fins up well but has lost his bright coloring and just swims slowly without much energy. Tried 3 different fish foods including some he used to love, and even tried frozen brine shrimp which he liked before. He seems to be fading away slowly. Any ideas what's wrong with him?