I've had more than a few chewed tubs. The biggest problem I've found is if you don't have the tub close enough to the wire mesh top. I think I had a 1/4" gap and that was too much, so I raised the tub so that it's only a 1/8" gap. That prevented most of the holes, but I have a set of three rats that can still manage to chew a hole in the corner. I let them have their hole that's about 2.5" in diameter, and they have never escaped.

The other problem I've found is that some rats will find out how to chew the small raised bump on the bottom of the tub. They seem to be content to just break through so the hole doesn't get that big, and most of the time a piece of duct tape on the bottom is all that is needed to keep it from growing.

Finally, one good way to prevent holes is to provide your rats something to chew on. I've been giving mine rawhide chews like you give to a dog and they seem pretty happy with them.