CO2 is a basic "need" of all plants. Either you provide extra CO2, or the plants can only grow as fast as the supply of CO2, from the atmosphere and from breakdown of organics in the substrate, allows them to grow. With the amount of light you have, assuming those bulbs are located right at the top of the tank, the plants are being greatly restricted in growth by the shortage of CO2. That makes them extremely vulnerable to algae attacks.
It is the amount of light you have that drives everything in the aquarium.