Maracyn not working on BGA

I've been dosing Maracyn for 5 days now (at first at half the recommended dose and in the last 2 days the full dose) but it seems to have no effect on the BGA. I can still see it everywhere and it still looks very green and spreading. I'm pretty sure i didn't buy the wrong meds... the back of the package says it contains 200mg of erythromcin activity.

When it works.. it should turn all the BGA in the tank brown-red right? Seems like everyone else has had results after just 2-3 days.