Public enemy #1 here.
There is no quick fix here. And ACEMOW: Good to hear you got rid of the sea hare. Would have died in your tank, glad it was a rental. Bandaids are not the fix. Time and care will make it all better!
Ronny, my new reefer friend, as it sounds like you are away a lot, I would say your hermits are the problem. If all params are ok, crabs are opprunistic killers. They will eat other hermits, kill snails for shells, and fight for food. I hate to say it, but let the hermits kill eachother off, and go snails to clean algae, nassarius to eat the waste (bristleworms are great too), and maybe even a bristle star if over feeding is common.
Aside from that, water changes w/RODI only, rinse the food in RODI, and have the equipment up to par.
Happy Trails! (smileyfacething)
***were not that bad up here in our igloos! (anothersmileyface)