Hermit crabs & turbos

I had a couple blue legged hermit crabs and about a dozen turbo snails. 
They seemed to do a great job cleaning up.  After a bout with ich, and me
over medicating the tank things seemed to come back almost to normal.  A
couple weeks after putting the charcol media back in the filter one of the
hermit crabs died (no biggie I say) then in the following week the turbos
start dying and then the second (and last) hermit crab got a sack that
covered the opening to his shell, and so did the last couple turbos.

The water chem was normal.  no ammonia, nitrite, little nitrate (7ish), sg
1.023.

The sack on the shells, is that pretty much decomposition?  it started white
on the hermit crab and turned a burnt orange when I figured it was dead and
not doing  something new.  If figure the same for the turbos.

Any comments?

TIA,

Paul