Part of the clean up crews job is to condition the sandbed, if you have one. The snail poo give the bacteria something to eat so it multiplies and they nip algae problems in the bud. I'm not sure why people think half a dozen snails is enough to be effective in a 50+ gallon tank. The different types of snails perform different work in different parts of your tank, some for glass, some for rocks, some for sand...they all have a job. I depend on my Nasarrian Army to take care of my sand...it's amazing to see them pop out of the sand when I feed the tank.