The comet goldfish get to a pretty big size, over a foot. So they aren't suited for a 15 gallon aquarium. Also are you running a filter on the tank? I would recommend rehoming him if there isn't a bigger tank in the near future. Also if you can answer these basic questions theyll help keep your fish healthy, I'm sure our familiar with these since you've keeping fish for 8 years
Are you getting a bigger tank soon?
Do you have a filter?
Is the tank cycled?
Do you use dechlorinator?