There's already been lots of great advice given in this thread, so I'll just throw in 1 more point that I don't think was mentioned. Most tanks that I've heard about with an algae prob have a PO reading of 0. Once your algae gets to a certain point, all PO which are being introduced to the tank thru feeding/fish poo tends to be consumed by the algae before it reaches detectable levels. Additionally, when you try starving the algae out by not feeding the tank, or running the tank with no lights for several days, the dying algae releases PO which continues to feed the other algae in the system. It becomes a vicious cycle. Its fixable... in my experience, it just requires patience.
Every tank and situation is different, but for me, if I start to notice GHA or Cyno it's b/c I've been slacking on water changes, or it's time to change out the bulbs. If I've been on top of my water changes, it's always been time for a bulb change. Regardless of the age on the bulb, a minor shift in the light output seems to really kick start the algae growth. For me, this was more of a prob for me with PC's than it seems to be with MH's.
You may want to try this. Do a large water change (ie...30%), run the tank with no lights for 3 days, and at the end of the 3 day period, do another large water change. Also, I'd swap out the bulbs. I know that you stated that they only have 8 mos of use on them, but all bulbs are diff and maybe, just maybe, they're output has already shifted to a point where it's contributing to your problem.
Just my .02,
Mitch
(Someone feel free to step in here, and enlighten me if I've misspoken).