Why are my hermits dying????

[saltwatersucker]

I am very new to all of this so any info will help. I have an 11 gal reef tank with live rock and sand My tank is cycled and the levels are good I have a Pulsing xenia, button polyps and a mushroom all of which are doing very well also have several snails and feather dusters and all are doing great also. The crabs take a couple of days to die upside down with there legs wiggling upwards. I have tried to turn them over on the sand or put them on a rock but nothing seems to work I have lost about 8 Crabs this way.
PLEASE HELP!!!!!

 

[dc]

Could be anything? Too high salinity, poor acclimation, even something killing them.

[welcome]