If possible, you should try and medicate a single fish in a hospital tank all by itself. Most folks have a spare 10 gallon that they use for this reason. You set the tank up when you need it. If a hospital tank is not possible then you must treat the whole tank. It should not hurt your other fish. It's just not good practice to expose them to medication if they don't need it. I don't think it does, but you will have to be extra diligent in checking your ammonia, nitrites, and nitrates when medicating your tank, because some of the meds actually kill your biological filter. You should be able to get the pima fix and the melafix at any PetSmart, Petco, or any large chain. I would call my local mom and pop store first and give them the business if they have it instead of a chain, but sometimes this isn't possible. Good luck, and let us know how it turns out!