If there were a problem with a glass tank, I would think it would be the lip on the bottom of the tank. Most tanks are made with a 1/2" or larger lip, so the flexwatt would not be in contact with the glass at all. That might make it inefficient for keeping temps where they need to be.

With a tub, the bottom of the tub is in direct contact with the flexwatt, and thus the temps are easy to maintain.

Gale