You have an RO filter? Have you looked into bacteria/carbon dosing or Granular ferric oxide? All of these will help alot w/ nitrate/phosphate, and the algea that goes along with it.

1 powerhead isnt going to be adaquate for this tank. More flow can only help. If the flow doesnt keep nutrients suspended in the water column, they will not get removed w/ the protein skimmer, and will settle to break down in the tank, causing more NO3 and PO4.

Old, used light bulbs will also grow algea pretty well. Is your light fixture new or used?

So far though, looks like you are on the right track for info to getting this tank into shape! I too like the keep it simple aproach.