I'm trying to keep shrimp/ crayfish in my paludarium. They look healthy when I add them, but die overnight. I just got a crayfish yesterday as I thought the problem was over as I got rid of flatworms in the tank. The crayfish was doing okay yesterday and this morning, but within 30 minutes went from active and healthy to not moving and curled up. What could be the cause and is there any hope for my crayfish?