daniella3d is right, you can most certainly have Cyano bacteria in a system with an undetectable nitrate level. There will be nutrients in any tank and it doesn't take much to feed the cyano. If the bacteria is absorbing the DOC's before they can be skimmed or absorbed by carbon, GFO, etc then it can thrive. Vodka dosing will bring a tank to a ULN state but it will make cyano grow like wildfire. I had an outbreak a short while ago and tried everything I could thing of, at the end of the day it was Chemi Clean that cleared it up (I also pulled my biopellets and replaced with GFO but I won't be so bold at this point to say the pellets were the cause or contributor).
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