okay i setup my tank and its gone through its cycle...i bought 4 hermit crabs and 1 astrea snail and 2 turbo snails...when i put the turbo snails in though, they quickly digged into the sand and stayed under there...its been 24hrs and they still havent came up...each time i dig them up and put them on the rocks they do the same...the astrea snails and the hermit crabs are doing great though eating forever...is it usual for the snails to do this?