a cuttlebone in the tank would likely get gnawed on. They are dirt cheap if you buy them in bulk. You have to take a butterknife or some other dull thing and chip off the brittle backing, then drop in a chunk of the chalky bone. Almost every turtle I've kept has gone after cuttlebone eventually. It's almost pure calcium, and they can't eat too much of it. For species that require it, UVB is still needed to metabolize the calcium, but I'm not sure about the UVB needs of snappers.