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 delhezi, 7 gal betta.