Snail's shells empty and slim covered...?

[Key Equine]

Anyone know what might be causing my snails to die?

Over the last month or so I have lost five or six snails. I find their shells empty and covered in a thick nasty slim. It is a fairly new tank, about three months old. The levels have been steady at ammonia 0, nitrite 0, nitrate 5 for the last two months or so. The sg is 1.023. Temp is 75 degrees. The snails were nassarius and trochus.

Any ideas? Thanks!