i had one t5ho on my 55g, it was 18 inches from light to substrate, I had no problems with out co2. I should have been running co2, but i never got my solenoid to seal right. Just be prepared to read about par (which you seem to already have started) and be able to move the light up off the tank. You could ask around to rent a par meter too, someone in my city does it so i don't see why there wouldnt be on in the DC area.
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