I believe that it is probably safe to label this as being cyano, but second opinons would be great! I do have some questions because I'm not sure where this stuff is coming from. I have 0 ammonia, 0 nitrites, 5ppm nitrates, 0 phosphorus, and yet this stuff persists. One interesting fact is that it is completely gone by morning (when the lights are out), and then slowly builds to this level throughout the day. My lights are PC, about 6 months old. Does anybody have any ideas? Oh, yeah, my flow is about 1800gph in a 155 gallon tank. thanks for your ideas!