Update....
While flashlighting the 120g last night, I found a small crab in a hole in the rock with a small hitchhiker snail in its grasp turning it over and over
Snail diameter was less than a 1/4". Did not see the whole crab, but I imagine its back would have been about 3/4" in diameter. Tried to stab the crab with some tweezers, but the b****** climbed out of the hole and took off
This crab was waaaaay on the other side of the tank from where the graveyard was. Don't think it is large enough to tackle an astrea or margarita, but got thinking I may be dealing with a larger snail-killing crab. So I set up a crab trap near where I found the small crab with a not yet dead, but not very mobile astrea that's been in the tank for the last two days
While setting the trap, I had to move some smaller pieces of rock to support the trap and found an astrea shell picked clean.
No crab in the trap this morning
Will move the trap to a location closer to where the graveyard was tonight.
Also found an astrea snail on its side at the back of the tank near the crab. It looked like it was trying to right itself, so I reached in and set it upright. It crawled away like normal
I believe I may be looking at two different things here. One may be that the snails are falling down and are easy prey for the predator, possibly a crab. The other may be that there is a big crab in the tank killing astreas and margaritas. I know for sure there is a small crab taking down little hitchhikers snails, though
Perhaps they are the same crab?
Further investigation is certainly warranted.