I have almost exactly the same tank, a Waterbox 70.3 (24x24x20) and had the Hydra 32 over it. I rented a PAR meter and took some measurements and they were a little low for what I wanted running that light at full power. I bought the Hydra XR30 G5 and took the same measurements and at 60% it was about twice as much PAR.
That light was a little more than your $700 but its an awesome light so far. Just my opinion.