Was gonna ask about your source water too. Just because nitrates and phosphates don't show up in huge numbers in the tank, doesn't mean the problem algae isn't being very efficient at using it up
If your bulbs are old, consider that as well, also getting your Ca and Alk in balance, and keeping it there will get the good coraline algae going and help diminish the bad.
And of course there is skimming, and there is SKIMMING....
