Honestly, this is just paranoia. Snails can serve as vectors of diseases that are passed on mammals especially humans wherein giant African land snails (GALS) are known to pass the meningitis disease however I don't believe it is a common occurrence for snails to pass on pathogens to fish. Take ich for example. Ich will eventually die after days of isolation of snails from fish. They don't rely on snails as hosts in order to survive. A quarantine of a week or two should be enough if you are still unconvinced.