Wasser is (as usual) right on. If you put a ton of light over a tank, the plants will quickly use up available CO2 and nutrients due to accelerated growth. If they are pearling, that usually means CO2 is ok, and a lack of Co2 usually won't harm plants...just keeps them from growing as fast, as bushy, etc.
You might start with some root tab fertilizers, like sea chem makes. Then start looking into liquid ferts for the water column...like Flourish.