Research.

A Google search reveals that what's inside of aquarium thermometers is a combination of alcohol and red dye, not mercury. So even if the "mercury" had busted, Mr. White would have been OK. But broken glass in a tank is never good... so he got a few extra gallons of fresh, clean water for his trouble.

I moved the ping pong ball out of his tank that night because he was pushing it so hard, he gave himself bruises on his upper lip area. I also ended up moving the blue skiky plant out because he was moving it by grabbing the top of it and attempting to impale himself on it. Neither of those behaviors are acceptable.

I used some aquarium sealant and rocks to weigh down the bottom of a less pokey plant and dropped that in his tank. He's been happy with that as a toy and moves it all over the place. He also enjoys flashing up against a ping pong ball floating in the top of his tank.

Given those activities, I think it's hilarious that if I feed him when his tank light is on, he is scared of the food and cowers under his rock until the light goes off. Then he'll come out and eat his food and carry on with his activities. As an FYI, his tank is located near the kitchen, so it receives external light throughout the day.