Hello, I just put 2 of my Neon Tetras down because they had a disease I couldn't cure. I tried 4 different medicines and nothing worked. The sole survivor of my tank does not appear to be sick, but he will be if I don't add more tetras for him. My question is how do I know if the tank water is safe for new fish to add? It has only been a day since he lost his friends but time is of the essence. I appreciate any help I can get. Thanks!