Yes, excessive use of GFO can cause RTN. Using a fast acting GFO like Rowaphos and bringing the PO4 down too fast also causes RTN.
When using GFO in a new setup, always start 1/3 of the recommended amount and slowly increase the amount each week.
If you use GFO when you don't actually need GFO you will have all sorts of problems with your SPS. With that said, I utilize GFO in all my reefs and have great SPS results.
If you have a good export mechanism (water changes and good skimmer), reasonable stock list, aragonite sand and/or porous live rock, the tank will take care of the P for you. The great thing is that it is always in equilibrium with "just enough, but not too much" as long as you don't do something dumb. It works, really. I guess that vendors cannot sell patience and ecosystem.
My guess is that it will go down to where you cannot even test for it... eventually. It can take a while for the bacteria to grow that were being held back without food - maybe 3-4 months.
If you started with dry/dead rock, please PM me when it gets down to zero - I am doing an informal study on how long it takes for tanks started with mostly dead/dried rock to become true ecosystems. My current hypothesis is about 20-24 months, but the more data, the better. If not, then thanks anyway.
Anyone have any bad experience with that. .
Can share ....