@kobain72 , I agree with
@Hanuman . Purigen, as well as any action aimed at decreasing dissolved organics in the water is a good thing, a.o. for preventing algae. But it's effect on cyanobacteria would be probably very meager.
Strong oxidizing agent (hydrogen peroxide is perhaps 'purer' than potassium permanganate) works very well when applied
locally against
small cyanobacterial colonies on sand. If cyanos already infested the whole tank, don't do that! I've tried (and perhaps many others). Before you erase cyanos, you make too much damage, essentially you destroy the tank.
More promising ways: Change the composition of your water substantially, esp. nitrogen and / or phosphorus. Also, you can vacuum as much cyano as possible and then cover your tank in complete darkness for three to four days. Then vacuum the rest.
These methods sometimes work. Truth is, cyanos are formidable enemies. Good luck!