my friend has four rbp's that size and he has a book that sais that if your fish are constantly swimming beside each other(even while turning in circles), and tend to try to keep away all other fish away you probably have a breeding pair. That area would be considered their nesting site. This will be no help if you only have the 2 rbp's. Pleco's, electric blue lobster's, snails and other hard shell species are able to survive in your aquarium. 
