HELP!!! i think my fish is dying!?

So i have two fish that i got from a pet store. I think they are tetra fish, maybe not though. Lately one of my fish has gotten extremely skinny (even though i see him eat) and has turned a darker color. He has also been floating at the top of the tank, belly up, but then when i tap the gl to make sure hes alive he just swims away like nothings wrong with him. I've had these fish for 6 years, and i had a third but it died a few years ago. does anyone know whats wrong with my fish? Please help!