Some turtles like more animal-based foods and some like more fruits and vegetables. The sliders and cooters like a good amount of fruits and vegetables. The smaller mud, musk, map, etc. turtles eat mostly animals. Painted turtles are in between. I feed my rebellied turtle (a cooter) a variety of animals and plants. I give fruits and vegetables every other morning. See
http://www.fishpondinfo.com/turtles/turfeed.htm for lists of possible foods to try.
Commercial turtle pellets are supposed to be a complete diet but I think variety is the key to good health. I suggest trying some of these things for your cooter that mine likes: mealworms, waxworms, grated carrot, grapes (cut in two), kale, other greens, etc. as well as a number of commercial foods (I have like 8 kinds).