No, Melafix should fix up the goldfish's fins and its not that harsh of a med. What size tank do you have? also what kind of fish and how many of each do you have?? Im sure the Koi didnt hurt the goldfish on purpose, healty fish are able to go a week or more without eating if need be. Maybe your Koi was trying to breed with your goldfish and thats why its fins are all hurt. Seperate the goldfish into his own container and treat him with melafix and that should do the trick.. Good Luck.. and i hope your fish gets better.
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10 Gallon
60 Gallon Tank
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