Hey people! I've noticed that my fish tanks are a little drab for my two bettas, and neither of them like the shelter I've put in their tanks. After looking around the house for a while, I found a ton of these big, beautiful shells, and I would like to make something for my fish with them. For a reference to size, the picture is the actual size of the shells.
Here's the catch though: they came from my great grandmother's house, so I have no idea where they're from, or even if they are real shells. Knowing this, how can I make sure they are safe for my fish?