I was concerned about one of my snails this week- the horned nerite in the main tank. I hadn't seen it for several days, then finally saw it on its back under the heater. I moved it, placed upright, and watched it for a while. It did not move- for a day, then another. I lifted it out of the tank twice to check- no bad smell, and when gently poked it slowly closed the operculum. So it's still alive. Now it's started moving around again, slowly.