The tank is in the basement and above the tank are vents where the cats walk on. I think he jumped off of one of the vents just on the perfect spot either directly onto the brace or onto lid close enough to snap it. Not sure how old the tank is exactly but we think its near 20 years old. The Canton question is almost 13 pounds and jumping down a few feet would be a lot of force on an old piece of plastic that was thin to begin with.
Is the black bit plastic or glass?
If it goes where the clamp is fitted, then it's probably just a cross member and you can glue a new one on. Get a piece of glass that is the same thickness as the tank (6, 8 or 10mm), and silicon it across the top from the front to back. Let it dry for a week and your good to go. The cross members are usually 4 inches wide.