What other fish could work in my aquarium?

I have a 30 gallon aquarium with a marigold swordtail, a blackskirt tetra, 8 neon tetras, and an otocinclus. It has bright lighting, medium water flow because of a strong filter, and all fish are happy and content, although I have snails that I just can't get rid of! I have a small rock with a hole in the middle that a fish larger than the blackskirt tetra can swim through. The rocks are white, and are pretty hard, ie not good for a fish that would want to dig. I also have three artificial plants, two of which are good for shelter, and one real stem plant that is on the verge of death because of the snails. What fish might I want to get?