if your trying to grow plants with 20 watts of light in a 40 gallon tank, I think your in for an unwanted suprise. The only thing I could think of that would be able to barly survive would be java fern, or maybe some kinds of moss'. with 0.5 watts per gallon, you plants will take VERY little or no fertilizers in your water collum. Do the plants grow at all? I would emagin that your algy is comming from the fact that most plants can't grow properly with 0.5 WPG, but the algy love it. Mabey someone can correct me if I'm worng, but I think your plants can't compeat with the algy for ferts in your watter collum. The algy can grow in just about any light, but plants don't. Injecting c02 won't help the algy untill you upgrade your lights.
75 gallon, fillstar xp2 with inline reactor, 12 Lb pressurised co2, 4.2 WPG-corallife 3x96 watt PC-, fluorite/silica-sand substright.