Hello; I have had the maylasian trumpet snails (mts) in my tanks for several decades. I like them so it was not an issue for me. Within the last year or so they began to dissapear from my tanks to the point that there were none. I am not sure why.I had those trumpet snails in my sump, annoying as hell because they'd block the drip tray! Dunno what happened, but one day there were none. And hasn't been any in months. Never seen one in the main tank either (dead or alive, and i just removed all the sand).
The only clue my be the growth of what I call slime algae. It may also be what is known as cyanobacteria-algae. The type that forms green sheets on surfaces. For a time the sheets of algae would not form on the gravel surface. When the algae sheets began to form on the gravel, I also began to notice the lack of mts. In the past when the lights came on in the morning there would be some mts on the tank glass. I have no idea if the two things have any connection in terms of having the mts dissapear. Something else may have reduced the mts and once gone they were not arround to eat the algae.
I have been tearing down, cleaning and setting my tanks back up for many weeks now. I found some mts at an area fish store and introduced them. Time will tell if they will restablish.