This may seem like an odd question but will ich or marine velvet be able to survive in a system where the fish have become immune to these parasites? This might be a good question for @Paul B to chime in on. I had Ich in my tank and was unable to remove my fish so I opted to treat the tank with a "herbal remedy" I kept up treatment for 5 weeks as directed and in the end lost 3 of 11 fish in the system. Of the survivors my Yellow tang was the first to show signs and ended up with bubble eye due to a bacterial infection. Yet he is now doing quite well. It's been well over a month now that my tank seems to be free of ich... or is it? Can I risk the health of other fish by introducing them into this system?