Some people take a twin T5 balast and mount the tubes wherever they fit under the existing hood. Others like me take the hood off completely and put luminaires over the tank. If you have fish that jump removing the lid can be problematic although a new lid can be constructed if you'd a little handy.
Vallis has always grown well for me even when my lighting was only 60W over 220L. I suspect an iron rich substrate helped. I have struggled to keep it to the back and sides of the tank, each week there are new sprouts growing more forward and central where the evening sun catches the tank.