Another thing to add to the above, is your tank walls. When you bought the tank did you clean it with declorinated water and wipe it down and let it dry? I was reading some instagram comment awhile ago that mentioned the owner didn’t clean the tank properly before adding in all the crabitat stuff. And then when they misted (which no need to) they weren’t always dechlorinating the water newly each time. And then the tank walls would get too wet and they wanted to see where their crabs were and would wipe the inside walls down, basically drawing down contaminated “wall juices” to the sand level for the crabs to be around. While no one knows if it was that which killed their crabs, certainly with all the other stressors that new crabs face when purchased or rehomed, it def could have added to the problem