Cherry shrimp will do quite well. Or you could try daphnia or californian black worms. I put some of the latter in my hillstream tank last year and they love hiding under the stones. They seem to be doing well enough although I am certain quite a few get eaten by the fish. You can combine them with shrimp as well but there is a possibility the shrimp out-competing the worms for food. Shrimp are great detritus eaters and a big population will dwindle even a snail population.