Agree that your basic problem is too much light. Cutting it in half will help in the long term but you will still need to clean up and remove as much algae as possible. Trim any leaves that are infested or dying, remove dead leaves floating in the tank and toss any plants that are too much of a mess. Spot dosing with Excel will help kill the BBA. Twice weekly water changes will also help while you get things back under control, you will need to keep your filters and tank clean. Keep plant mass trimmed and as stable as possible so that your circulation is consistent.
Are you using pure RO/DI water or a mix with tap? If it is pure RO you need to reconstitute the water with something like Barr's GH booster so that you have enough calcium and magnesium.
Is your CO2 system pressurized or DIY? How are you monitoring levels?
Even with half the light you will still have a lot of light over the tank, I suggest you look into using dry ferts. Liquid ferts like the Pfertz and Seachem line are expensive and not very potent, they are better for low tech tanks.