I can't figure out why. Snails, clams, and everything else is fine. It must be something they're eating in the sandbed, but I haven't made any changes except some new fish.
I found one of my fighting conch shells empty a week or two ago. I figured one of these happened: Either my other fighting conch or my queen conch killed it; It wasn't getting enough to eat; Or, it got stuck under a rock or something similar.
But in your case, the first and the third thoughts would probably be ruled out. Are all your snails fine?