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ive had my red ear since i was 2 my dad got it for me he said it was only about the size of a quarter when he got it and now its about 7 inches well anyways ive been feeding him reptomin pellets all his life and ive never tried to feed him anything else is this a bad thing and also can i give him chicken i read some where u can but what kind dark are white meat and cooked or non cooked thanks

Chicken is only used to entice turtles to eat that have been ill or not eating. Other than that white meat chicken (boiled, skinless) can be given occasionally as a treat. I'd skip it though if I were you and concentrate on the veggies/plant matter that he should be getting daily instead.

A mature RES should get pellets every other day, once-a-day, the amount that would fit inside their head if it were hollow. A variety of veggies should be offered daily, especially since a full grown RES is more herbivorous than younger RES. There's a list of what to feed and not to feed at the top of this section. Try feeding dark leafy greens such as Romaine, Green leaf/Red Leaf lettuces, dandelion leaves, etc. along with carrots, squash, pumpkin, zucchini, sweet potato, and red peppers. Even if he doesn't take to veggies right away keep offering them. Sometimes you have to apply tough love and withhold the foods they are used to for a few days until they begin eating what they should. The lack of the vitamins and nutrients from the veggies may show up eventually in his body, but it's never too late to make a change for the better.

Older turtles need veggies more than they need meat, so if you give it, use it only as an occasional treat. Definitely cook it. You can boil it in water.
I'm not sure about the white meat/dark meat.