There are alternative medications that you can use on sick fish, other then those that you put directly into the tank. Baths, injections, and medicated food can be used to treat seriously ill fish who are not responding to store-bought medication. None of these, especially the baths or injections, should be used without consulting a vet first.

Injections
Injections put antibiotics directly into the fish's blood stream, and therefore the success rate of using injections is very high. There are downsides though. Handling the fish will cause stress and you will probably need to have the fish sedated (see below for instructions on how to sedate) before attempting injections. It takes a certain amount of skill to do injections properly, therefore you should consult a vet, or even have him/her do the injection for you.

 

Anesthetics (sedating fish)
It is sometimes necessary to sedate fish, whether it be for treatment or for examination. While generally you can look at the fish to see what is wrong, you may need to sedate the fish in order to properly examine skin, gill, or fin problems. I really do not recommend sedating fish without a professional present.

Medicated Foods
Right off, your at a disadvantage if you use medicated foods because there is no way to make sure the fish eats the food. Sick fish generally don't eat very much. Its possible to lace ordinary food with antibiotics, but this isn't really recommended, because the medicine may rinse off in the tank water. They do have already-prepared medicated food, but its only available through vets.