Nassarius Vibex snails are vital for DSB tanks, as they consume detritus on the DSB. Excess food, fish excretions, rotting plants are all consumed, within reason. They stir the sand with their large 'foot', and travel very fast when they sense food in the water! I've also observed them on the glass and rockwork as well as in macroalgae stalks, so they pretty much cover your tank well. Excellent part of your clean-up crew.