01-22-2013, 08:22 PM
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If that is gray in colour [photos do distort colours a lot] then it is most likely a type of brush algae. I see it in some tanks. You can't avoid it, but you do have to keep it under control, and this is with light duration.
Tank light is critical. If the light is too intense for what the plants need, it will cause algae. If the intensity is balanced, then the duration has to be considered. You start at some number, say 10 hours, and if algae is increasing beyond normal, reduce by an hour, wait a few weeks, if not resolved, back another hour.
But, remember than in new tanks during the first 3 months or so, the biology is still establishing and algae will take advantage, so don't be too drastic as it may be somewhat temporary. This can be somewhat offset by having lots of plants, and heavy floating plants.
Window light can make a difference. I learned two summers back about this. I wondered why every summer the brush algae increased, then I realized that the tank lighting was just on the border, and the increased brightness and duration of sunlight during the summer caused it. I kept the drapes closed all summer the last two years and no more brush algae increases. I have my tanks in a fish room, so this is easy to manage.