Turtles Need Calcium

 Calcium
  Turtles need calcium to build healthy shells and bones.
  A cuttle fish bone in the tank will be nibbled on by most turtles and
  supplies extra calcium. Get a new bone, when the old one get slimy!
  There are also calcium supplements specially for reptiles. 

* Vitamin D3 
  Synthesized in the turtle`s body using natural sunlight. A Reptile light with
  the appropriate ratings for UVB is recommended,
  or a supplement can be fed. There are supplements
  available that contain Ca/D3.
  Of course, the best way for your turtle get D3 is to get
  unfiltered sunlight, and then it will synthesize the vitamin.

* The calcium to phosphor ratio of foods is important, as the more phosphor
  there is in the food, the less calcium can be absorbed. 2:1 is a good ratio.

To find out more about this click here.