I asked the same question not to long ago...
I have a 37 gal tank and 5 hermits, 5 snails, 1 bristle star, 1 pepermint shrimp, and a cleaner clam but I was told that I needed at least 10 more hermit crabs and 15 more snails in my tank...
So I'm guessing for a 30 gal tank, if you've got a lot of LR to clean up, you should probably have at least 15 hermits and 15 snails....
you could probably try a lawnmower blenny too. They're fun to watch and they eat up a lot of algae...