Originally Posted by Perry the platypus
Mom said it is a 22 gal. I know it's too small but they usually bunch up. I don't know if it's cycled or not; I'll ask mom when she gets home. Water parameters nope I don't have. I think it is common goldfish.
Are you referring to the shortened bodies of fancy goldfish? Or are you referring to the painful process of stunting?
You need to test the tank for ammonia and nitrate as it sounds like this is fin rot caused by poor water quality. Goldfish are very salt-tolerant and you can add 1 tablespoon per 5 gallons to help in the healing processes. You will certainly need to start doing weekly water changes on this tank, and if those are already happening increase them to twice weekly as this was caused by poor water quality.