i have a 29g its been up and running 4 years. my water parameters honestly couldnt tell you. i go to the fish store about once a month and they test it. it's always good though. i only have 2 clowns and some inverts.
I agree hermits probably just fighting for the best shell. I recently added a bunch of new shells for my crabs and they all fought over one shell. Slowly they all found new homes but I think a couple lost the battle.
I would bet that the snails have been rising up against the crabs and eating them... or not. it could be any number of things. they could be starving too. you may also have a mantis shrimp getting a feast.