After weeks of living my second Trocus snail has died... There's plenty of brown algae to eat and they've been doing a great job of it, but for some reason they just die.
did you drip acclimate the snails before you drop them in? I read that the snail's internals are very delicate, the slight shock in pH or salinity or nitrate level can damage them. They won't die immediately but organ damage will cause a slow death.