I think my comet is bleeding internally.

I think it was his food that did it. He was just fine this morning until after I fed him (I feed him tetrafin and switch it out for other brands to give him a little variety). Almost immediately after being fed his fins start clamping up and he spent most of the day at the bottom of the tank in a corner.

he's had a small red patch at the tip of his fin for a few days, but I just assumed it was a bruise, (He's had one before and the red disappeared after a few days).

I'm assuming there was something in the food that's poisoning him. I doubt he has septicemia because I do water changes every week to two weeks, (that and he's the only fish in the 15 gallon tank). I came back after work to check on him and the red has just spread in his fins. I think he might die at this point but if anyone's got any ideas on how I can help him or make him more comfortable, I'd appreciate it.

I really hope he pulls through b/c I've had him for almost two years...

What's weird is I'd just done a 20% water change a few days ago.

Anyway, the good news is I think he's going to make it (probably got more worried than I should have b/c he's rarely sic). I did a 50% water change last night like you suggested (thx for the help, btw) and gave him some anti-bacterial fish food and his fins seem to be clearing up now. They don't have that pink look to them anymore (their isn't as much red in his veins either).

He's swimming up to me like he usually does instead of laying in the corner so I think he's getting back to his old self. Fins aren't clamped either so that's a good sign...

I don't want to feed him that tetrafin anymore b/c the reaction came so soon after he ate it. I'm not sure whether it was the food or the water quality...

Is it possible for a fish to have food allergies?

Probably a stupid question. But I'm a relatively new being a fish hobbyist.

>>> If I am not mistaken comets get pretty big so 15g seem a bit small. What kind of filtration do you have?<<<

He's about 5 1/2 inches if you include his tail. I'd like to build a pond or buy larger tank but I'm in the process of selling my house so that's out of the question for now. I have an aquatech power filter with bio-fiber filtration.