I had the same issue few months back. Bought some snails and crabs, acclimate and into the display. Snails refuse to move, crabs into hiding. All dead in few days. I was asked by fellow reefer whether i used any insectide sprays and yes, i do. Made 50% water change, and a carbon air filter for my skimmer. No more issue. Skimmer pulls in a lot of air, including contaminants that will accumulate in the water after some time.