I have a 4ft tank with 2 x 3ft flouros and two 15 watt incandescents which are on all the time to prevent the shock when the flouros come on automatically at 5.30am. At the moment the flouros are on 15 hours/day. Light intensity is one factor but the depth of water is more important in my view. I have a growlux in the front and daylight in the back. Works OK for me. The incandescents are on dimmers but at present are on full strength---looks like moonlight at night. My tank is 6oomm high. The plants have to be pretty well established to get away with 15 hours/day, I have worked up to that.