I use (right now) two 36 inch, 30 watt, coralife actinic 100 percent true 03 blue in my 215g tank during the daylight hours, nothing at night, my fish love it as I have a swfo tank. I dont like using white lights during the day, my fish seem more peaceful with "purple" during the day, I prefer a blue tint but have a purple tint with these bulbs. I will use four bulbs in the future but right now my tank is cycling and I don't want all that wattage in the tank for fear of algae brown diatom blooms.