Vitamin D is a very important nutrient in the diet because it enables the animal to absorb calcium from the digestive tract. Under the best conditions, a pet turtle will live outdoors where it will be exposed to sunlight, a chemical reaction goes on in the skin that provides Vitamin D to the animal's body. So most animals that live outdoors will probably be fine, and the owner will have no reason to supply Vitamin D as a supplement.
But animals that live indoors will need a Vitamin D supplement. Dr. Scott cautions us to remember that window glass and aquarium walls filter out the beneficial sun rays. A Reptisun of a special UVB light can provide Vitamin D producing rays. However, if the bulb is more than 8-10 inches above the animal, it does little good. Also, even if the light bulb has not burned out, after about six months, the bulbs loose their UVB producing effectiveness.