I disagree completely with the "clean up crew" advice. You don't need a single snail, shrimp, crab or starfish in a system for it to be healthy. If you have detritus building up on the sand bed, you don't have enough flow. If food is left over, then you are feeding too much.
Whatever you put into a tank must have food to sustain it. Adding a multitude of snails and crabs will force you to add more food for them. It's either that or you will need to "recharge" your cleanup crew on a regular basis, which is just a nice way to say "buy more livestock to replace the ones that starved".