to start go with some turbo snails and astraea snails (although you may have to monitor them as they can't turn off their shells) trochus snails are a close relitive but can right themselves when flipped over, i have noticed with mine though, they are usually hiding during the day and do most of their grazing at night time.
once you're system has a little more stock to it and is more established you can add some hermit crabs, shrimps, clams and what not, but for a newly started system stick to 5 or 6 snails just to feed on the diatom bloom