Hello. So a few weeks ago I got a comet goldfish from petsmart. (I know, not the best place for fish) The one I got looked very healthy, nothing on him to suggest that he was sick, as a matter of fact the second I got him home he was very active in the tank. The tank he is in is a 10 gallon (another bad on my part, waiting for my 55) but knowing his tank was over stocked I do very frequent water changes, I'm talking like every other day, sometimes every day if the tank looks especially dirty. It has a HOB filter, with a bit of cycled media in it from a previous tank and a sponge filter just in case. For the first few weeks he was doing amazing, but suddenly a few days ago he just started to have clamped fins, like all of them, top fin, the little ones on his undersides, all but his tail and his side fins really. I don't know why he has this sudden change in him. He was very healthy, the ammonia levels and water quality is fine, I'm just not sure why he is acting so sick. He is not as active but he still moves around the tank when I come close because he is waiting for food, which I guess is a good thing because he still wants to eat, but Im not sure what I'm doing wrong here, I know he's not in a right sized tank but I'm sure with keeping everything perfect in this temporary home he would be fine. I really like this fish. I forgot to mention that he is about 1 1/2 to 2 inches long so he is just a young fish. Also I just call him a him.