How do you get low calcium if your feeding cactus pads and grass witch are high in calcium and you have cuddle bones out for the torts ? But I would think in answer to your question soft shell weak leg joints .
A friend of mine recently lost 2 babies to calcium deficiency. They received plenty of UV...but the diet lacked calcium rich foods and was never supplemented with powdered calcium or cuttlebone. By the time they passed, they were like soup in your hand...
Feed a good, varied diet and add a supplement periodically. I like to sprinkle some MinerAll when I feed Mazuri.
I free broadcast organic limestone flour on my grazing area. This way they actively consume if as well as it is washed into the soil to be taken up by the roots of the graze which is then consumed. Here on the west side of the cascades we get lots of rain so our soil tends to be mineral weak. This helps correct that.