Cyano can fix atmospheric nitrogen, it does not need an organic form, that's why it can thrive in a nutrient poor environment. If you have a little bit of time a little elbow grease can get you through the cycle, sometimes though you need to step it up, that question will be answered in a few weeks. Do a little work and if it seems fruitless we'll walk you through an eradication schedule, but that is a last ditch effort.