First, and by far the most important, reduce your light intensity. If you drop the intensity into the low light range your chances of evading algae go up significantly. The next problem is that the plants will be structured to use CO2, although a CO2 ladder doesn't give you very high CO2, so they will have to "regroup", switching their enzyme production to optimize their "engines" to work with less CO2. Some plant species will not be able to survive with low light and no CO2, so you will have to do a lot of removing of plants that aren't surviving, and perhaps planting more appropriate plants. It will amount to a pretty significant re-aquascaping job.