Yeah, it could be that he's just old (he was an "office" fish that my GF took in, and then I took him and put him in a 5 gal tank instead of his old 1 gal bowl).
Are there any tests for this sort of thing? The tank has full filtration with a bio wheel that I just changed 2 weeks ago (the filter). PH is at 7.6 and all other levels haven't changed (I don't have my records since I'm at work right now).
The only thing "wrong" with the tank is algae (bright green stuff that won't go away!).