You do not need a "CUC". I've kept many tanks over the years with little to no snails and/or crabs at all, and had no problems. What do you think is going to eat off of the sand bed and what exactly is on the sand bed that you want eaten? You probably just need an increase in flow to keep detritus suspended in the water column and that will solve the problem.
Remember, whatever you add to the tank will need food to survive. Why do you think people keep replacing dead snails and crabs? They starve.