Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Depending on how far above the tank the lights are mounted, and reflector design/efficiency, the light burst may be exceeding the max photosynthetic limit for the plants. Light in excess of that doesn't help anything but the algae.
Stay with the current 8 hour photoperiod for now.
Verify your CO2 is really at 30ppm or so during lights on, preferably with a drop checker, to double check your controller settings and probe accuracy. Make sure adequate flow is present, especially at the areas where you're having algae problems.
You will see different opinions on CO2 at night, none of which really predict if it's necessary to help eliminate algae in your tank specifically. So I invite you to simply try it instead. Put an aerator on a timer and run it at night, not so much to drive off CO2 (the controller with compensate anyway), but to add oxygen; this may not be necessary either, but it's a sensible precaution to eliminate the possibility of oxygen depletion.