Gfo is like any other product, the amount you "need" to keep phosphate levels at a certain set point won't ever be determined by tank volume.
It will be determined by the rate of addition of phosphate. You could have 10000 gallon tank with three fish and brand new ceramic rock, and you'll probably need less gfo to keep phosphate at "reef" levels than a 300 gallon tank with 40 fish and 7 year old rock. Whether "less" means smaller volumes changed more often, or huge quantities changed every 3 months, the amount you need is dependent on the amount you use. Just like carbonate or calcium.
Gfo can certainly reduce phosphate levels. My... Question (I guess?) is whether gfo can reduce phosphate levels to the point where the extra iron, in some cases, doesn't negate that benefit.
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