If they are smooth rocks no shape sides like real river rocks I would say a Bn Pleco. They love hiding in rocks and cleaning them.
How big is the tank. What kind of water flow does it have a strong current? What temp? A hills team loach could be a good choice depending on the temp of the tank. Depending on size a group of Garra could be good they come from rocky areas.
You can't go wrong with snails.
80gal planted Angelfish community.
100gal SA cichlid, Silver dollar, and delhezi. No plants!
36 gal Planted Swordtail community.