I know. My friend asked me to take his fish, and one of them turned out to be a lone buenos aires tetra that bullied all of my other fish. The goldfish is a tiny wild variety that I caught in a nearby goldfish pond which I am using to teach my sister how to care for fish with. Anyway, back to the subject. The platy's breathing, swimming motion, and appearence are fine, but he seems to have a punctured swim bladder or something, because he sinks. The neon tetra died, but I think it was from some unrelated cause, since it was extremely fat and had a large colorless patch on its side this morning. What do I do?!
EDIT - I don't have ammonia, nitrite, or nitrate test kits.