cheers for the reply but i did not make my question very clear
my room is about 6x7 feet the tank is only three but i veg my plants to fill the room and am getting about 15/18 oz at a time with just 1 600 the i bend and strech and prune my plants to fill this space and some times the tops are really pulled down so in all 2 plants cover atop area of 6 x4ish 6 feet is a long way for one light to cover and keep close to the plant. as the middle of the tank space always produces the best i wanted to put more light at the ends so is it better to move the light up and down [which may be the cost effective way ]or stick 2 400 in