Welcome to AA!
What kind of snails are they? Where is the beach? US? East coast, west coast?
Snails aren't carries of the parasite ich. You noticed 1 fish had a problem before you introduced the snail. I would say the snail had nothing to do with anything. Missing scales sounds more like a bacterial problem than a parasite problem.
How's your water quality? Can you post some parameters?
, temp, nitrate, pH?
How long have you had the fish and how long has the tank been running?
How often do you do water changes?
You never want to treat the fish while in main display tank. Always take the fish out and treat them seperately in a qt setup.