Maracen 2 and blackhair algae

I have alot of cynaobacteria (black hair algae). I use the prescribed dosages of Maracen 2 - about one week. I took out the carbon from the filter before starting the medicine. At the end of the medicine, directions say to do about a 1/3 water change. I did that. Unfortunately the black hair algae was almost untouched. I got to thinking and I use purigen in my canister. Could that have blocked the Maracen 2? If you don't think so, what do you all think prevented the Maracen from working?