10-10-2010, 08:35 PM
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This has the appearance of cyanobacteria, not a true algae but often considered as one. It is caused by organics. Though having said that, I find it occurs in certain tanks and never in others, even though under identical light, same feeding, fertilizing, etc. I would do a larger weekly water change, 50% of the tank, and vacuum the gravel thoroughly. This should easily come off and be sucked up. I won't go into possible "treatments" as they are in themselves risky, and this will only return if the organics are not dealt with.