I know this might not be what you want to do, but since there is no dry wall up and the tank is on the floor, couldn't you take out the stand framing you have done and turn the boards the opposite way and regain some space? I know it would be some extra work and cash, but it is better to bight the bullet now and do it, then regret the limited amount of space you have after the tank is up and running and drywall up and painted!!
Just a thought!
If it were me I would turn the 2X on their vertical that the tank will be sitting on, and install the vertical supports 90* from the way they are now, and you would have A LOT more space! Then put some cross bracing in the main frame where the tank sits and it would be on the money!
Like I said it would be extra work and $, but it would be soooo much easier now!
Either way this is a COOL BUILD, and I am jealous BIG TIME!!!