I'm down to three (from five) Malaysian Trumpet Snails over the course of a few months. I originally had two of them in a ten gallon with celestial pearl danios and sparkling gouramis. They would spend most of the time on the glass at the surface of the water until one day they wouldn't come out of their shells. They stayed in their shells for about two weeks so I figured the danios might be picking on them and I put them with the other three in my five gallon tank with yellow shrimp and micro crabs.
They moved around for a bit, but now won't come out of their shells again except for brief periods, they'll hang their bodies halfway out all curled up, and I've noticed the "foot" appears to be turning orange/white. The two died after I moved them, though I've had few problems with the shrimp and none with the crabs.
In both of these tanks I'm using iron slag as a substrate. Is it possible that this isn't appropriate for snails to be crawling on? Perhaps the grains are too rough/sharp?
Bi-weekly 20% water changes
No CO2, no fertilizers, nothing with copper. I don't have a harness test at the moment, but the water is hard enough to leave some pretty bad scale on the tanks if I let it evaporate past the trim.
Here are some pictures, hopefully they illustrate what I mean about their behavior.