I just received my new lights from Catalina. So now I am wondering how far above the tank i should mount it. The tank is 75 gallons and 20 inches deep. The light system has four T5 54 watt 6500K bulbs (two of them are "plant growth"). The two bulb types are staggered and have there own on off switches. So what do you think? 3 or 4 inches above???