My turtles super feeding frenzy?

I have two red-eared sliders, one 6 years going on 7, and another almost a year old. During the past 2 weeks they devoured almost all of their use-to-be-feeder fish. These fish we have had in our home for a few years, some got really big, the two turtles never use to touch them. THen in the past few weeks they all started disapearing, they would kill and only half eat the fish. I remove the fish that have been in the tank for a day incase no one gets sick. But i put in 38 feeders yesterday and they nearly ate all of those aswell. theres only a few fish we have in there and one of them we had over 2years. Why sudden frenzy now, should i keep buying more feeders so they dont eat big ones? I dont know what to do, please help!