I've got a rainbow shark and he doesn't do much in the way of tank cleaning - not like my little Otto's do. Even my red tail shark (in a different tank) is better at nibbling away at tank debris/algae than the Rainbow. My rainbow seems to prefer eating bloodworms, live brine shrimp, flakes, freeze dried daphnia etc. Maybe he's just been spoilt too much and it's made him a lazy worker!
If you're looking for something to help keep algae etc down then I wouldn't bother with a Rainbow Shark, to be honest. They are also quite aggressive fish. In fact, any of the "sharks" can turn a peaceful tank into a war zone.
Depending on what fish you already have in your tank, you might be better off getting a few Otto's. They are tiny little things, very cute to watch - and real little workers. They are amazing at keeping algae at bay.