Is there any data that shows HC GFO does indeed have a higher capacity. You use half the amount. does it have twice the capacity?
It might, but I've not seen the data to support it.
Also, does GFO remove all PO4 and then reach capacity, or does it remove slowly over time? I'm trying to ask if it is like a DI filter. You get a pretty distinct line of what is exhausted, then it breaks though. Is GFO like that?
Not exactly.
GFO binds phosphate in relation to the amount in the water. At 1 ppm it will bind more than at 0.1 ppm, which binds more than at 0.01 ppm. The binding is a reversible equilibrium.
But, at 1 ppm you may saturate all binding sites in only a few hours or less, while at very low concentrations it may take a long time to saturate the GFO.
Smaller amounts changed more frequently sounds fine.