HCA: Hermit Crab Association

I have 11 crabs in a 40gal breeder tank. They are mostly around half dollar size, with a couple that are a little larger, and some that are smaller. The biggest one went down for a molt first, and then they all soon followed, with the last 2 going down about 2 weeks ago. The big one was down for more than 2 months, and I had an empty tank for about a week.
The big one finally resurfaced, but he keeps disappearing for a few days at a time. How do I know he's not burrowing around under there and eating other crabs.
That is what I am worried about.
Temp and humidity are good, lots of good food, salt water, fresh water, (Poland spring), 100lbs of play sand mixed with a bag of eco-earth coco fiber, sprayed everyday for a good "sand castle" consistency.
I think that my tank is big enough if half were molting, and half were not, but I am also worried that maybe it's too crowded down there, if they all molt at the same time.
I think that I am set up pretty good, but I am worried what is going on down there.
thanks for any help

I don't think you have anything to worry about. After a crab molts and re-emerges for food and water, he'll keep going back under to rest and gather strength. The molting process takes a lot out of a crab so the need to tunnel under and chill is normal.

Thank you Crabinski