Jesus....I have been battling algae for a few months and couldnt understand what was happening. I would pull the cup off the skimmer and there wasnt much skim...but there were bubbles so it looked like it was working. It is an older Tunze 9001 and always worked great. Well I changed the pump about 6 months ago and I guess I had broken off a shelf inside when I was installing it. I guess that shelf was critical to its operation. So I cleaned the skimmer and fixed the shelf. I wasnt getting much skim at all so I ordered a tunze 9004. Well....between the 2 day shipping...the old guy started cleaning the shit out of the water...lol. I changed it to the new one anyways...its more powerful....hopefully after the break in I can get this algae in check...it already looks better.
Skimmer = critical equipment IMO.