Yup! that would be my guess. cool water and hot glass doesn't mix.Could have been from heat.
I had similar thing happen to me once - was in process of cleaning and moving tank, had it sitting outside, long story short - i got sidetracked and by the time i got back to the tank 2 panels had cracked - simply by sitting in the sunlight.
i should have said there was no water... just glass.. would heat have affected just glass that much?
cracks are 75% along the bottom, starting from the front, then elbowing all the way to the far side, i dont think bottom pane could be repaired