SOIL DECOMPOSING AND CREATING ENOUGH CO2? oh i doubt thats the case. anyone who adds co2 knows that you would need to have ALOT of decay to achieve enough c02to equal how much we pump into a tank. I'd shut the lights off for a week wrap the tank black it out. If your algea is out of control that will put a big hurt on it. go and plant the heck out of the tank with the fastest growing plants and with the lowest demands you can get your hands on. 50% aged water changes faithfuly every week then back down from the over planting. algea and your plants both thrive for nutrients and algea unfortunately grows fastser then most plants UNLESS the plants don't leave enough for the algea to thrive. when and if you see the plants whiting out withering then you may think about ferts