Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
How much GFO to run for your volume, and when to change it.
Run GFO in a reactor with gentle tumbling, not a packed bed and not a violent boil. Too much flow grinds the granules into iron dust that blows into the tank. Too little channels the water past it.
grams = 284 L × 0.3803085 g/L × 1 = 108 g
Granular ferric oxide binds phosphate onto iron oxide, and its capacity is finite. Manufacturers publish a quantity per volume of tank water rather than per amount of phosphate, because the rate at which a system produces phosphate matters more than the standing level.
The published rates differ between products because the granule density and iron content differ. Bulk Reef Supply high capacity GFO is quoted at a lower mass than their standard granular for the same job, which is the point of it.
Start at half the standard quantity whenever phosphate is above about 0.2 ppm. Fresh GFO strips phosphate fast, and SPS corals respond to a sudden drop by bleaching and losing tissue. The slow route takes weeks and keeps the corals.
Tumbling matters. The granules should roll gently in the reactor. A packed bed channels and does nothing, while a violent tumble grinds granules into fine iron oxide that ends up in the display and on the corals.
GFO does not release phosphate when it is exhausted, it simply stops taking any up. So the signal to change it is phosphate starting to climb again rather than any particular number of weeks.
GFO removes silicate as well, which is useful against diatoms, and it can strip other trace elements at high dose rates. That is one more reason not to run more than needed.
Phosphate bound in rock comes back out for months after the water column drops. A tank that reads zero with GFO running and 0.3 ppm a week after it is removed has a rock problem, not a water problem, and only time fixes that.
Sources: Bulk Reef Supply GFO