Megalodon teeth are fine for tank decorations. They should, however, also have an epoxy coat to prevent enamel peel. Here are a couple I pulled out of a desk drawer. I'll see if I can find my bigger teeth later.
The 2 bottom brown teeth were collected in Callao, Peru when I was participating in a counter-narcotics op.
The 4 outer darker teeth were collected on several dives in a Florida river. The center tooth was collected from an overburden pile (beach restoration project) just outside of Damneck, VA.