leaving feeders in there will just add ammonia (toxic to fish-from fish waste and other decaying crap)
if you want to do live feedeing i would say once a month but quarintine them first (keep them in a seprate tank for around a month and have them in meds that will kill parisites)
better food options include: shrimp/prawn, talapia, catfish, whitefish,cod, pollock, salmon (not more then once a month since it would make them fat)
beef heart and other organs (once a month or less- same reason) as salmon,pellets.....