last week i purchased about 20 blue legged crabs to help stir up the sand bed alittle, about 2 days later, they completely devoured my chromis but thats another story, now, most of my crabs have died or are in the process of dying. they seem to be leaving their shell before they die. any idea of whats causing this? all readings are 0 but ph is a tad low, around 7.8. temp is 80.