You have to condition them to a feeding response. Start with as many small ones as will fit you tank, feed everytime you open the lid, don't over feed, when you do feed feed only live food and one feeder at a time, stopping while the rbps are not yet sated, gradually increase the size of your feeders as the rpbs start to feed as you want them to, and expect to lose some rbps as they grow. In a 150g tank start with around 15@ 2", by the time they reach 9" you should have about 1/2 left and they will eat a feeder a little smaller than themselves.