I have a tank much like yours Tantrix.
It is 6 feet long and almost 2 feet deep with a mere 16 inches front to back. When I filled it, the tank bowed out so much along that long 6 feet that I drained it rather quickly to remove the stress of all that water. The glass had bowed out more than an inch in the middle of that 6 foot span. What I did next was to create a simple frame from 1x2 inch lumber with a couple of front to back cross pieces at the 2 foot and 4 foot points. I actually created a mortise and tenon joint where the cross pieces joined the front and back pieces of my frame. The end result was quite simple. I slipped the frame in place over the glass, with no real attachment point. The cross braces kept the frame from slipping down over the glass too far and kept the glass from bowing outward. I cannot measure any deflection of my own half inch glass now that I have that frame on my tank.
I would not simply fill your tank unless it stays in proper shape when you do that. Having a glass front or back blow out due to excessive water pressure load is something I would want to avoid. I am sure there are tank manufacturers that will sell the frame pieces if you are willing to pay their prices. I have seen such things for smaller tanks at one of my LFS. The price was quite high to buy a plastic frame that might contain $0.50 of plastic. They were asking about $20 for that simple piece of plastic. In the UK think of it as 50p of plastic costing 20 pounds when formed into a tank frame.
Why not simply build a frame out of cheap 1x2 lumber and stain or paint the final product a color that you like? It will be strong enough to hold your tank in its proper shape, will be a nice complement to your tank and its cover, and it is something you can do with minimal tools and even minimal woodworking skills. If you build your own hood, you can even use the frame you have built to give you a perfect fit and a perfect match to that hood.