Thanks for the response Evil! and wow you're close by! That sounds somewhat similar to what I was thinking about.. going to 6 rows

After thinking about it for awhile, the 72 number seems nice since it would only require 6 drivers... but stepping up to 8 wouldnt be too bad if I wanted to go to 96.

for the 72 approach I could place two 6x6 grid pendants with 3" spacing on LED center, 10" above the water with 60 deg optics.. each LED should cast a circle of a little over 11" diameter on the water surface which also means that it should fill in (5.7" radius) the sides pretty well.

My concern with this approach, however is that I may end up with a dark area in the center of the tank where the two pendants don't overlap and / or the brace interferes. I could move them in a bit but I would probably be hurting the outer edges.