I cycled my tank with the lights one...but they were just fluorescent bulbs... 35 watts or something. If I had to do it all over again, I would still cycle the tank with my lights on, even with my VHO's or metal halides. I'd just rather get the algae bloom over with :) If you are cycling with a fish or 2 in the tank you might want to give them a sense of day and night. Another thing to consider is that not all nitrifying bacteria grows in the dark... there is beneficial bacteria that needs light to thrive. Hope this helps. Good luck!