Hey
Ok, so you have eliminated phosphates. Now, what about silicate matter, DOM, nutrient levels, old bulbs, unfiltered H2O, poor skimming -which can all contribute to what you are observing. To improve your situation, and ensure a better and more helpful response perhaps list more about your system. Get as detailed as you can, your bio load, feeding schedule, filtration status, lighting regiment, water change maintenance, anything you do to the tank would help nail down your problem. Hopefully others will chime in and give you their OWN FIRST HAND EXPERIENCE, and help you out.
Regards
Thorium