How much GFO?

[asylumdown]

wouldn't the amount you need and the frequency with which you replace it depend entirely on the amount of phosphate present in your system? I don't think there's a hard and fast rule for x amount per x gallons. If you add a small amount and you have lots of phosphate, your GFO will be exhausted within a matter of days (hours?), and will just be a bunch of rust tumbling in your reactor until you replace it, leading to very little visible difference in your corals or algae growth rates.

If you have almost no phosphate and you add lots of GFO, I doubt anything will happen in your tank, as the levels won't really change all that much. It will just take longer for the GFO to be exhausted.

I think people get in to trouble when they have high phosphate and add lots of GFO, in which case there's a massive, sudden drop. Whether or not this is true for you can be pretty easily determined by testing.