Hi Andrew,
Sorry to hear about your trouble.
My advise is that you found out exactly what your sick koi has and cure it. Don't waste your money on medicine after medicine. You are just stressing out your fish even more.
Talk to the dealer you bought your fish from and see if they have any problem. Most likely, they will too if their fish is the carrier.
I myself don't like melafix too much. "Quick cure" is my preferred one.
Also, make sure your salt level is not too high (0.30 is good)
Prazi and proform are both good medicine and EXPENSIVE!!! So make sure they are applicable before using them at all.
BEST MEDICINE OF ALL= QUARANTEEN NEW FISH
