I'm having the same problem in my 55 gallon tank! It's soooooo gross. My amano shrimp eat it, but when I have a bloom of it (like I did a few days ago), they can't keep up with it. I've been getting help at other forums for it, and the things that have been suggested are lights too bright/on too long/getting too much natural light from a window, adding CO2 would help get rid of it (you said you are injecting CO2, so that's probably not your problem) and they also told me to change the water more often (but that's not your problem, either, because you do 50% weekly).
So I think it could be your lighting. Your bulb wattage should be ok, but the length of time you leave it on could be a problem (it should be more like 8 hours), and/or you could have direct or bright sunlight on it sometime during the day.