GFO is Granular Ferric Oxide. I use PO4x4. It reduces your phosphates pretty fast and is rechageable (which is great because it isnt cheap). There are other types of GFO, but I find them messy...need a lot of rinsing. Biased because I used PO4x4 from the start.
Google GFO.
For a quick fix you can use a liquid phosphate reducer if you have a skimmer.
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