I use both!I give extra calcium for my females and baby's daily without d3 twice a week with a light sprinkle with d3 you must be careful not to over supplement with d3 or vitamin A, since it stores it in the body and doesn't pass out the extra like other vitamins do so if use to much it can be harmful.
From what I read in different tortoise websites, outdoor tortoises you use the calcium without D3 because they get the D3 from the sun and for indoor tortoises use the calcium with D3.
I live in cold Illinois, and there is a lot of wild life in my back yard, not safe for my Burmese star babies so I can only keep them indoors for now. I use calcium with D3 and also TNT from Carolina pet supply.
Who else uses TNT? Seems to work well because my babies are huge.
Im from Illinois as well I raise and breed greeks,I only use proven products by the pro's,Zoomed supplements and lighting for year's they are the best on the market today!
Also, to add, I believe I read that D3 from the sun, you cannot overdose on, whereas the D3 from a supplement you can because it does not metabolize the same as natural sunshine D3.
Aha, here it is: "You cannot overdose when getting your vitamin D from sun exposure, as the body has the ability to self-regulate and only make and use what it needs".
Remember everyone: Adequate vitamin D3, phosphorous and magnesium intake is necessary to enable proper calcium absorption. They all work together. When there is insufficient of one or the other, the imbalance will cause problems. Which is why we should strive to feed a variety of foods, to cover all bases. And sunshine to kick it in! Variety for vitality! : )