i would imagine since you are posting in the planted forum... you have plants?
if so.. what plants? the type of plants you plan on keeping will make a difference in your light requirements.
I also have a 90gal planted.. im running 2 x t5ho 48" life-glo bulbs... 6 hours on/ 18 off
"Never argue with a idiot, they will drag you down to there level then beat you with experience." "One never needs their humor as much a when they argue with a fool."