One of my zebra nerites I noticed had been hiding a lot recently. The other day I did a MAJOR trim and realized huh...he's not actually in the tank anymore... I found him under the aquarium stand, he must have climbed out. His operculum is closed up, and he doesn't stink of dead snail. I plopped him back in the tank, the first thing I noticed was that his shell is slightly buoyant. He's on the substrate, but he's half floating and after being back in the water I haven't seen any movement all yesterday and I didn't look before I went to work today. Is he/she dead most likely?