I have zero explanation for this, but I run a brs dual reactor with high cap GFO and ROX carbon. I've run it 3 or 4 times and EVERY time I have done so, within a day I start to get more cyano, and it keeps going till I take it off line. Within days the cyano is reduced. as been the odd additional other variable it COULD have been, but I'm going to wait a week or so, turn JUST that on but make no other changes and if it happens again, there is NO other explanation. I change out 4.25G a day on my 330G system and that is the ONLY other thing from last time. I've stopped the reactor, and the water changes, and things have cleared up considerably. Like your last comment, if I find absolutely that was causing it, I will try to just run the carbon to polish off the water.