I have a 20 gallon tank. I had an eel catfish for over a year. A few days ago I added a rope fish and a snail. Unfortunately both fish passed rather quickly. As I removed them I noticed white spots on both, which from a little research suggests worms? I have no other fish in the tank except the snail. Can those worms survive in the snail or outside any host at all? How should I treat the tank so that this doesn't happen again when I get another fish?