On GFO being expensive - I would research running Lanthanum in your tank. There are a few articles about trying this in a reef tank. It precipitates phosphate, requiring you to set up a filter to catch near where you run it. But it supposedly makes phosphate export easy and dirt cheap compared to GFO (Lanthanum is commonly available for swimming pool treatment). That said, it hasn't been used for a really long time and there are risks. I was going to go that route with my old rock, then decided to just acid wash them and start over.