Not that you probably haven't already found something by now, but I'll let you know what I use on my 29, 30 gallons. I initially bought several blue leg hermits from a local fish store. They are the most vibrant crabs of any that I have seen and they do an equal job to my red legged and jade legged crabs. The jade legged crabs devour turbo snails if they flip over though. This being said, I would go with approximately 10 blue legged crabs and 10 cerith snails for the sides of the tank. Nerith snails are round and lay small white eggs ALL OVER the tank. And most likely they will never make it to be adult snails due to filtration and lack of macroscopic food.
