My best guess is the bubbles only show then there is water in the tank because the weight of the water is pushing the tank sides out, exposing the weak seam. I wouldn't buy it unless you plan to take that front panel off and re-install it (no small job and not for people who have never done a tank build).
Just my opinion. BTW, I've built 8 tanks over the last 10 years... not all for me.