Hi, Basically I had 6 female cobra guppies, 4 male guppies, 6 cory catfish, 3 weather loach and a few babies. I was unaware of my dad adding 2 more cory catfish in the tank last week, I was on work leave. Today I came home and I find 1 cory catfish dead, like his whole body was white, pieces were missing from him etc... I assume he was dead in the tank for quite some time now and wouldn't be surprised if he was for a few days now. What is the best thing that I could do right now to avoid any of the other fish dieing? I know he could've been sick and spread a disease in the tank, and I already had to take out one female guppy due to a white mouth and frozen gills before she affected the others. Any advice to treat the tank?