When the temperature of the water in a red-eared slider's environment drops below a certain temperature, the turtle's metabolic system slows down and he loses his appetite. RedEarSlider.com states that this is one reason that it is important to keep water temperature closely regulated. Sometimes, red-eared sliders also become reluctant to eat in the winter if the room temperature in the house is too cold for them.
According to RedEarSlider.com, red-eared sliders prefer to be fed in water. If you need to entice your turtle to eat, the site suggests that the food be bright-colored, strong-smelling, or alive and moving; this is so the turtles can easily detect the food. Some examples are mealworms, bright-colored feeder fish, tuna fish or even tadpoles. Sometimes, if you drop plain or bland food into the tank, the turtle either will not notice or have no interest in the food. Red-eared sliders love to hunt, so food that is alive and will attempt to swim away is a huge plus.
It is also possible that you're overfeeding your red-eared slider. RedEarSlider.com says food should only be offered once every other day. Another advantage of feeding live prey, like a small group of guppies, is that you can watch how quickly your turtle consumes them and determine how big his appetite is. If he leaves one or two feeder fish uneaten, chances are he is well fed. RedEarSlider.com also notes that during the winter, the red-eared slider may have less of an appetite. Feeding will not have to be so frequent at this time.