I'm treating my goldfish I've had for 9 months in a 10 gallon quarantine tank. He developed red streaks on his fins in my 36 gallon tank after introducing two new goldfish that were quarantined for two weeks prior to introduction into the 36 gallon. His fins began to rot away. The guy at the pet store gave me tetracycline to treat him. I put the tetracycline into the tank and the next morning the goldfish was on his side at the top of the tank just breathing.
I stopped the tetracycline immediately and moved the goldfish to the 10 gallon quarantine tank. I added maracyn-2 according the the directions. It has been 5 days now and although he periodically will swim around, most of the time is just spent floating at the top, limp, and breathing. I'm not sure if he'll get better or not. He does eat a little bit.
I'm keeping daily tabs on the water quality with my api master kit and it all appears good. I bumped up the temperature a bit and added a bit of salt to the water per instructions as well.
The two goldfish have since died in my 36 gallon tank as well despite the water quality being spot on. I now have 3 albino coreys and one zebra danio that seem to be doing fine in the 36 gallon.
I just don't know if my goldfish is going to get better or if he's suffering for no reason. His fins have seemed to have stopped rotting away since treating.