Folks seem to to get vexed dealing with black hair algae. Is the green any more benign? I have a few wisps showing up in one tank, should I be A) mildly concerned or B) panic in the streets?
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Can you post a photo of the algae? I'm wondering if this is not black brush algae which can be very dark green, dark brown, grey, although it is a red algae technically. Not that that really matters--if this is a planted tank, you want to have any problem algae well under control. Without plants, it is your call.