I don't leave Epsom salt in the tank because I only want to treat the ill fish. However, in looking through my notes, I see that 1/8 tsp per 5 gallons is recommended for dropsy if treating the tank. I have never done this though so perhaps someone else can offer more advice on it. You will see different dosage suggestions from different people. The problem is that there has never been an actual effective dose decided. I hope all goes well. Best of luck!