All good suggestions. I don't have any experience with overdose of phosphates, but is it possible that organic molecules are gathered near the surface, like in a saltwater protein skimmer? I know that organics do collect at the surface.. I would think they are hard to remove. Someone suggested using the vacuum upside-down to get wateer from the surface. Vacuuming the gravel might help in the long run as well. I think a water change is a very good suggestion, taking wateer from near the surface.
Oh, and put in activated charcoal (in the filter). It may or may not help the problem, but in this situation its use is certainly warrranted.