Marina, how is the tank doing? How is the phosphate issue? Unless you you do as Der ABT, you will just have to continue to use the GFO. I would probably get high capacity GFO. Unfortunately it does mean weekly changes and the cost issue.
I take regular photos of a few selected coral in my tank and noticed that colours worsened at a certain point. I looked back through my logs and discovered that it was right after I added a small selection of rocks to my sump. The other thing I noticed was that I was fighting a long phosphate battle, which I first thought was due to my fish load and feeding. So I was feeding less for many many weeks and the phosphate wouldnt really go down.
Like you I was using lots of GFO. A couple of weeks ago I removed these additional rocks from my sump and BAM! My phosphate levels dropped to less than 0.03ppm. I know its not feasible to remove all your rocks, but I think eventually the rocks have to stop leaching.
You can buy Lanthanum Chloride and use a 10micron filter sock to treat selected rocks in a vat or something...but its hard work.
Your corals still look amazing, and reefkeeping is just like this sometimes...but its worth it.
Anyway, I actually came and joined WAMAS because you have an amazing tank and you didnt have a build thread over at Reef Central.