For now I only have the one light, so that is the MH photo period. On my 180 when I ran 400s 7 hours was enough (but I did 8 trying to push growth, it was a waste of electricity). I switch to 250s and found 9 hours about right. So now it's 150s (I'm feeling a little inadequate) so I thought I'd start at 10 hours. I like the longer viewing period too
Most of the Acros are in the top 1/2 of the tank.