randomellis said:John,
Calcium (or any other vitamin supplement) with D3 has pros and cons. Pros: It contains D3 which is vital to the absorbtion of calcium and is produced by the exposure to direct, unfiltered sunlight or UVA/UVB bulbs. This makes it possible to keep a reptile indoors without having calcium and/or calcium absorbtion issues. Cons: It contains D3. It's a dual-edged sword. A little bit of D3 is great, but a lot causes D3 toxicity. Vitamin D is a fat soluable vitamin so the amount within the animal, if supplemented incorrectly, will build up until hypertoxicity occurs. The reason it is complicated to use any supplement containing D3 is that you cannot measure exact amounts if it is added to a mix, escalating the chances of over-supplementation and/or hypertoxicity.
My Opinon is to NOT use Calcium with D3, and expose your tortoise to UVA/UVB and natural unfiltered sunlight as much as possible.