Help with Hermit Crabs!

[raisemyrent]

Hello,

I seem to be cursed. Maybe my crabs I used to keep when I was in HS and had my 6 gal (no idea how anything survived, but it did) cursed me after I took the tank down.
This time around it's a standard 10 gal, and I seem to have gone through 2 crabs already. I bought a medium sized blue-legged one some months ago and it would not move from the one spot. I let him be for some days, until it was just the shell (literally). No idea what happened to him, or the body. I had drip acclimated him.

So what happened is that I bought 2 hermits about a week ago, a red leg and a blue leg. The blue leg kinda kept to his own corner and burrowed in the sand. Then he moved and continues to burrow. The one time, he was hitching a ride atop an astrea snail, so I knocked him down in fear that he was stalking him. It now happens to be at the front of the tank so I can see him moving his front legs, kinda feeding on invisible stuff, but won't move. The red legged got to business pretty soon: trying to jump over cliffs, etc. Then it was static at the one rock bridge for a while... about 2 days. And today, I found his shell on the sand just below that spot... completely empty (I picked it up). OK, let's assume he died, where is the body? is he gone moulting somewhere? There are NO other hermit crab shells wondering around, if you're suggesting he changed shells. In fact, I was meaning on getting some more shells this coming week, as the next ones over in size are well over what they would need (or be able to carry, I would think). These 2 guys are about 1/2" long, and my shells in there are about an inch or more.

Anyhow, any clues?

p.s. I tested and everything is where it should be (I actually never test). The tank has been running for just shy of a year, all corals and the 2 fish are happy and growing. I REALLY doubt there are any predators in there that have escaped me for that long and have killed nothing else. The yellow watchman goby is still suspected to have finished off the 3 sexy shrimp that disappeared mysteriously, but that is about it.
p.p.s I also bought 2 astrea snails at the same time as the crabs, and they're up to the usual with no reported problems. One even packs a lunch these days.

I've attached a FTS cause no thread is good without pictures. Bear in mind it's early morning for everyone in the tank, so they're just waking up. I placed the battery pointing up to where the blue leg currently lies. You can see the (now) empty shell just to the right of the brain.