Edit: i think i know what you mean, the brace in the middle is wiggly? Right?
The brace in he middle is to support lid you gonna put on the tank, as a matter of fact its wiggly posture is not going to affect the stability of the lid. Nothing to worry about. My 125 Juwel Rio got the same brace. And i believe that tank brace id the thing that is going round the tank top and bottom, the middle bit i call plastic strip.
omg im some sort of genius! I figured it out all by myself! I should be issued some sort of award here! Just a joke if someone likes bit of humour
It looks normal to me.. it's mostly designed to stop the front and back bowing once full and let them use thinner and cheaper glass.
The lid would likely sit ok on the top rim without a problem.
I would say to fill it up wherever save (like outside) you should be leak testing a used tank anyway. The brace part should become taughter once there is the pressure of the water.. at that point look for any cracks or anything, and let it sit a day or 2 to see for any potential slow leaks.