I would say the combination of low salinity and then the steady increase may have had something to do with it. Inverts, especially crabs and shrimp are hugely sensitive to salinity changes.

Whats "special blend"?

It's bacteria. Very nasty stuff - smells like raw sewage! :shocked!: Seems to work, though.

Thanks for all the suggestions. Guess I'll just chalk it up to the salinity, and keep my fingers crossed that whatever's left will be fine.

How about the possibility of them having starved? New tank, chemistry is well within the acceptable range, everything's growing and doing well - other than a few crustaceans... What's left of the hermit crabs all appear to be thriving now; but then I've also started feeding them directly. They're all ravenous.

my hermits are the same . have you tried pumps off for feeding? if you let some thawed food settle to the bottom they will find shortly. I have recently come to discover that my hermit crabs and blood shrimp really like scavenging for "new life spectrum marine fish formula" 1mm sinking pellet. my hermits have recentlly started to kill my margrita snails for their shells. apparently the 15 i put in there for them wasnt good enough.

There's pretty much no chance they starved seeing as they're all scavengers/opportunistic feeders. Even if you aren't feeding the tank (which you are) they'd still have access to food from the sand and rocks. It's much more likely your tank chemistry isn't as stable as you believe since the tank is less than a month old.