When I had these snails, they never came out when the lights were on. Afraid of the fish I guess... When the lights went out, it was like zombieland but with malaysian trumpet snails.
Yea, I considered that. I went out there with a flashlight around 2-3am several times and never saw a thing.
Based on what I've read, I'm going to say it's the Nitrate levels that caused them to die off, because I'm not sure what else it might be.
So far, Nerites & MTS have done very poorly in this tank.
Apple Snail, Ramshorns and pond snails have had no trouble.