Would start them at 10"-12" above at 5 hrs. Lower them daily until you hit 6-8" above the water and add 15-30 mins daily until you hit your desired photo-period. If you experience lightening during this period, raise the fixture vs reducing photoperiod.
For me, I relate the height of the fixture to the width of the tank. If the fixture is 18" wide and the tank is 30" wide I hang it higher - like 12" so there is better spread across the width. If the fixture is 18" wide and the tank is 18" wide I hang it lower - like 3" to take advantage of every bit of output I can.
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