Ummm...not to state the obvious here but you are combining a fish that soley feeds on urchins and crustaceans in the wild and putting it into an enclosed body of water with a large fleshy snail that seems to have no fear (or very stupid). No invert be it snail, crab, urchin or starfish is safe around a trigger. I have a tank with a Lunare Wrasse, Humu Trigger, Passer Angel and Sharpnose Puffer and just restock the "clean up (or out)" crew every month - snails, crabs and an occasional urchin or starfish. Personally I think it is good for them to eat LIVE inverts to keep their hunting instincs and wear down their teeth.
Good luck trying to save your doomed snails
