I did the same thing with my dying fish. I'm new to the game, just started out so my water treatment level is still low; I got #2 now. I don't like to read the spoilers so I blindly bought the unknown chemical. It transfered a couple of my betafishes into oriental betas. The could not thrive in my tank and were getting very sick. When the health meter was red and the fishes would only live to see one other fishy day, I sold them. Full price, no problem. Now, I play the game on easy, maybe this is filtered out in the hard way?