I run GFO on occasion to keep PO4 down, many around here do. Its best if you run this in a reactor, unlike carbon which can be run in bags. Carbon in a reactor is much more efficient, but can be run in bags if thats your only option or choice, passive vs active filtration.
GFO is something you really want to keep out of the system as far as dust and particles go, particles can get into/onto corals and cause problems. Carbon is more of an eyesore, and generally harmless if you get some into the system by mistake.
Some people do run both in the same reactor, but my own opinion on that is that they should be run in separate reactors due to the difference flow requirements. GFO requires less flow, just enough to gently bubble it or make it move in the reactor, whereas carbon you want more flow running through it.
The amount of GFO used is based on the size of the system, and I generally advise running at most half of the recommended amount to start, then work your way up to their recommended dose from there based on how your system handles it. Its possible to remove too much PO4 too quickly as corals do need some to thrive. The trick is finding a happy middle ground between just enough and too much.
