Guessing that it is 9 inches off the water (I think that is the arm height), I would choose whatever white you like, and then probably the blue between 80-100%. I think your tank is 18x18x18 and the max par at 20 inches depth is 100 (500 par at surface, 250 10 inches down) If you can rent a par meter that would be handy. You can also have it ramp to 100 for a few hours mid day and have it slowly decline and rise at night and morning.