Thanks, what color temperature bulbs do you use? Is 6,700k the optimal?If I remember right, the PAR on CF's go down to about 70% of efficiency after 6-12 months of use. If you think your plants will be fine at that intensity, keep on going. If you start having growth issues where there doesn't seem to be as much light in the tank, it's probably time for a replacement.
I usually replace after 6-12 months.