As the turtle become more mature they need more vegetables than protein. I've had my slider for 10 years, been to the vet for blood tests, and have a very healthy lady RES- and she does not eat feeder or any live animals-

I am also Buddhist and happy about this.

We keep leftover healthy food for her and feed her the pellets only every once in a while.

There are lots of sites that describe what to feed and not feed the slider. You can also try putting a piece of coral in their tank for calcium, and feed them sardines with bones. It's mostly the bones that the feeder fish are recomended for, not their meat.