I am wanting to change my Hissing roach and millipede tank into a desert theme. Add fake cactus plants and calcium sand for the substrate. If I do this, will I have problems keeping the hummidity levels up? Thanks!!
It depends on the lid for your tank. A piece of fruit that they're feeding on can provide a lot of humidity in a tank with no ventilation (glass lid, plexiglass, etc.). The contents of the cage are not as important as how often you spray or wet down the substrate, what foods and water you place in the cage and what kind of lid you have on it.
Calcium sand is designed to clump when it comes in contact with any moisture. This means that when your cockroach or millipede attempts to shed, its soft, wet new exoskeleton will become horribly caked with calcium sand, and it probably die as a consequence. Similar results are likely if the animal attempts to pass waste, lay eggs, or "birth" offspring in contact with the calci-sand; I can't tell you how many herps I've seen with it caked and hardened in their eyes and around their cloacas so I imagine the result would be the same. You also can't mist or wet down calci-sand, because it will turn into a clumpy mess. Overall, it is a ridiculously dangerous substrate and I wish it would be taken off of the market.
If you want a desert set-up, get a desert invertebrate. And don't use calci-sand, use a blend of playsand and soil.