What you need to understand is that if you have algae and you measure your PO4, you do not include any growth of that algae, you just check the amount that is unused in the water. When you start pulling out algae, you remove PO4 that was taken up by the algae BUT you also remove a source that consumes PO4. So what you did was replace algae with GFO which is a good thing but you forget that if you feed and things die off, they release PO4 over time. So you removed algae which will make PO4 go up over time, and you put GFO in and the value stays the same and you say GFO isn't working? It is, else algae would just grow back quicker or PO4 would go up instead of down.
So you need to up your game. Try to lower PO4 SLOWLY!!!!!! because else you will have a problem that your PO4 is too low. PO4 is buffered in the rock so it takes time to lower it. Keep pulling out algae because they are bad for the corals. Allow the GFO to pull it out or just add a double dosis for now to lower PO4 (you need double because you want to lower PO4 while you are removing algae which also lowers your PO4 so basically you removed one method (algae) to put in another (GFO) without actually increasing PO4 removal. So if it's stable you know the amount you need to keep you from growing more algae. You need a bit more to lower it over time. But it might drop suddenly once it's not buffered in the rock anymore.
