whats going on

I had 6 trochus snails and I found my hermits eating 3 of them last week but they didn't take the shells? And I found another shell empty last night? Maybe I have run out of algae? I suddenly don't have alot of algae developing in my tank and no more diatoms. I still have some algae on the bak wall but not much of it. All my other snails are doing good, except for one nerite (it was murdered for it's shell)? All perameters are where they should be.

 

Hermit crabs are opportunistic feeders so if the snails were unfortunate enough to be caught in a position where they couldn't defend themselves, they would have been eaten by the crabs. It has nothing to do with the amount of algae in your tank as the crabs also eat fish food.