If it's still a juvenile, try feeding it smaller meal everyday. When I say small I mean enough to diminish appetite, but turtle should still be beging for food. If you are feeding the commerical pellets, feed it the amount that would fit in the turtle head and neck. But do not rely upon the commercial food as staple; feed it varieties of food including a lot of veggies and leafy greens, which should make up the bulk of the diet. But younger turtle may be more carnivorous than herbivorous, but it doesn't hurt to offer them veggies from time to time. Adult turtle are more herbivorous, eating mostly plants, than younger juveniles and babies. I heard map turtle are more carnivorous than the red-eared slider, so feed it varieties of inverterbrates along with commerical pellets is the key. Again, do not just give commercial pellets to your map, but offer varieties is the key. I heard sometimes even the map turtles will eat veggies and leafy greens, so you can offer them to the your turtle if he will eat it.
Here is a diet summary for red-eared sliders; it should be something like this:
25% commerical food such as trout chows, dried cat and dog food, and good quality commerical turtle food such as Reptomin, Wardley, or Mazuri. Note: the commerical made specifically for turtle tend to be more balanced for them so I would use that more often than the other.
25% live whole prey such as feeder goldfish, pinky mice, crickets, mealworms, giant mealworms (or zoophoba), nightcrawlers and small earthworms. Occasionally, you can give them meaty item like beef heart, chicken, beef, and pork. All must cooked first to kill germs like salmonella, E. coli, and other food borned orgainism which can make your turtle sick. Note: I say whole prey because they are more nutritionally balanced than just muscle meat, which is reserved for occasional treat. Try dredging these food item in multivitamin and calcium supplement before given to the turtles. Calcium tablet can be hide in a piece of meat before given to your turtle.
50% leafy greens and veggies such as mustard, collard, dandelion greens, carrots, winter squash, escarole, green beans, sugar snap peas, snow peas, peas, parsnips, turnips, sweet potato, and yam, which should form the staple of your turtle's diet. Stay away from all the lettuces which is basically just water and cellulose, hardly any vitamin and minerals in them at all; this include all the lettuces like romaine, and red-leaf varieties. All fruits also have very little to no nutritional values whatsoever and are low in calcium and high in phosphorous, except of figs, which is high in calcium; fruits also have lousy caclium/phosphorous ratio, except for figs. When I say fruits this also include tomatoes which should be fed sparingly like anyother fruits because of them being nutritionally empty food.
