I feed mine Tetra Prima("Complete Food", a sinking variety of fish pellet food) and a variety of algae wafers and catfish pelets, frozen bloodworms and an assortment of anything else that'll sink (like prawns and mussels) or takes its time to get to the bottom of the tank- it also eats various types of fish flakes.
RTBS are not really scavengers, you should not rely of them to just scavenge food to survive, but instead make sure you are feeding them a variety of suitable foods. I'm not completely sure how large these fish get, but mine is about 5inches+ currently.