How big are your predatorial fish? And how much extra tank space do you have for a breeding program?
The easiest source of feeders in my experience is convict cichlids. They breed like rabbits, grow rapidly and can be fed as soon as they reach the size needed. If fed on a good rounded diet (and convicts will eat anything IME), they provide the added bonus of being gut-fed, reducing the risk of the fish they're fed to being nutrient deficient. A 20 gallon long tank with a pair and another for growing out fry can provide a lot of feeders. If you need more feeders, add tanks for grow out until you're growing out enough fish for your needs.
A 55 gallon tank with just regular old mollies can achieve the exact same thing, with the added bonus that you can acclimate them to saltwater and then feed them to marine predators.
Either of these will be easier and faster to breed and grow out than rosy reds. They will be healthier as well, as rosy reds, like goldfish, tend to have a higher fat content than either of the species I mentioned, which can cause health problems later down the road.
All that said, you should be doing everything in your power to acclimate your predatory fish to a good prepared food. Even feeders that have been bred with that specific purpose in mind are not good for long term maintenance and health. I cannot think of a single species of fish of the hundreds I've kept over the years that I could not acclimate to taking prepared foods, frozen foods at the very least and often a good quality dry food.
WYite
