Razor, you should have isolated the sick fish in a 5gallon $30 setup and treated the tank separately. Most aquarium medication contains copper, something that you will never get out of the tank completely - and it devours the bacteria (and any other invertebrates) that keeps the nitrogen cycle in balance. Almost any medication is going to disrupt the balance in the tank that can cause more trouble down the line.

Without that bacteria, ammonia levels can spike to dangerous levels - especially with how much waste goldfish produce. Goldfish can pretty much survive anything but not so much for already-sick fish.