I'm thinking the lights might be a factor? Back in the day when I ran NO actinics, I could always tell when they hit 6 months old because suddenly there'd be cyano in the tank. I'd replace the lights and it'd be gone in a matter of days.
How old are the lamps?
Could just be it just got a toehold in the sump and the light just happens to sustain it. What about shutting off the light for a day or two, is that an option?
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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