What to do about Fish Medication that killed all my fish?

i would contact API. clearly it was something wrong with the medication. the dwarf gourami is very hardy so i'm not surprised. if you have another small tank i would transfer the fish immediately. save what you can. empty the water and replace it with fresh. pretty much start over with fixing your tank. i use melafix medications and it clears up just about everything. very sorry this happened. its unfortunate. good luck to you and your remaining fish!