My nerite is usually very active and roams about the whole tank. Sometimes he disappears for days at a time because I don't restrict his access into the filter compartment. On Friday evening I spotted him on the front glass burrowing down into the substrate. He was about 3/4 covered. I thought he was not moving but after half an hour he was even deeper. I pulled him out and he has been behaving absoultely normal ever since. The only unusual thing was that morning when I refilled for the water change I noticed that he had remained above the water line while I was doing the change.