That is a 125 gal tank we are looking at?
Only thing I can think of would be to switch out your upper frame boards to a hardwood like oak and do away with the center braces all together. You would still have to use 2x6's and it would be pricey. Good news is that you wouldn't need a center brace at all. Seal the oak boards well prior to installation so that there will be almost no movement. And yes, you would have to change all of the upper frame boards out. Same species of wood expand and contract at the same rate.
My 120 sits on a 1x4 oak frame that I made with no center brace. If I had a longer tank I would build it the same way I'm telling you, only with 2x hardwood lumber throughout. I really like the idea of no center brace. You don't see this too often from manufactures because of cost. I looked at several AGA stands and was amazed at their assembly was done. Not nearly as strong as I would have done it.
I know the frame is not done, but I see a couple of other concerns. You need support directly under the upper frame. Right now you have the vertical 2x6s inside the frame. Add another one right at the corner so the weight is not supported only by your fasteners. And the fasteners.....those look like nails. Ditch them for 3 or 3.5" deck screws and glue every joint. You're going to have to become a furniture builder.