One can setup a small "bin" or "tank" that grows duck weed and that can be used to feed the fish. Also dog food can be used in a pinch, but fish were obviously never designed to eat dog food. Some of the dead or extra plant items like lettuce can be used to feed them as well. Hope that helped.
Tilapia were originally farmed to eat all of the crap from stripped bass farming. so yes tilapia will eat just about everything. this leads me to my next thought. maybe a tank or two with striped bass which has been a widely cultivated species. and just wondering for the sake of info have you considered talking to a few of the local restaurants to see if they would be interested in purchasing some of your adult fish? this would help with some of the overage of fish and also help supplement some of the expenditure you are having to cover.