This is not correct. Vitamin D can be overdosed given this way. The body stores vitamin D and has not regulatory means of limiting D when dietary. You may be confused with Vitamin D3 through UVB exposure. The body does self-regulate the conversion in the skin and cannot overdose that way.and they will only process what they can process of vD.
Another thing that worked for getting them lots of calcium fast was cooked ham. They go nuts for it and it doesn't mess up your tank. Those two things were the only supplements I ever needed.
Varied diet helps too.................but be careful of floaty sticks....I still swear they caused >10% of my dead turtles!
jeff