You may also want to consider cerith snails. They eat the sediment and extract the algae and bacteria. Here is a link to an old article that contains both cerith snails and conchs'.
http://www.reefkeeping.com/issues/2004-07/rs/index.php
You could likley replace your hermit crabs with nassarius snails. They will both eat the "left overs" but the snails will not kill other inhabitants.
***disclaimer: I hate ALL crabs, but for good reason.***