02-25-2013, 01:38 PM
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Others have correctly identified the issue and how to fix it. I've done a fair bit of experimenting with this problem which I have also had (but no longer
The first step is to reduce as much as possible the ambient daylight, especially if it is direct sun. And to illustrate how this can impact: I noticed for 2 or 3 years that brush algae (I am thinking this is probably what you are referring to) always took off in the summer. It finally dawned on me that the brighter and longer daylight could be the reason, so for the past two summers I kept the windows covered all day with both shades and heavy drapes. End of algae increases.
Second issue is the brightness of the tank light. Here, before considering different light (which would mean either T5 NO tubes as you now have HO, or a new fixture altogether), I would lessen the duration. I have my lights which are T8 and moderate, on for no more than 8 hours daily; any longer and brush algae increases again. You can go down to 6 hours daily, according to George Farmer. I would reduce down to about 8 and see if it helps.
You won't see the existing algae disappear, but the goal is to not have it increase further.