I have one, I may have mentioned that you should keep one because discus can get sick over just about nothing. But I actually just keep a supply of general fish medications, not just for discus. It never fails if the buggers are gonna get sick it will be when the stores are closed. I keep a medication with Metronidazole for Hexamita, Hole in the Head. I keep a bottle of formalin which is for flukes and your general fungus and ich treatments. I also keep an antibiotic as well, although I've never used it.
Forgot, I keep parasite clear as well.