The algae reduction process isn't just about phosphates. Reduction of the tanks photoperiod and reducing feedings using foods that have been rinsed first are where you start. You're already doing most of the things like RO
. There is no problem with adding a bag of phosguard, GFO
or even purigen. They will reduce the dissolved organics, including P4. Just add it in a bag to a high flow area of the sump.
It becomes a balancing act. Organic compounds need to be kept low to starve out the nuisance algae while not too much so you don't starve the algae in the corals.
Interesting side note, I believe natural reef levels of P4 are 0.007ppm. So why doesn't this starve out the algae in the coral? My theory is that our captive reef inhabitants depend mainly on photosynthesis from their algae. Not much planktonic life for them to filter. But in the wild, they get a lot more nourishment by filter feeding and they aren't so totally dependent on light. So that makes our captive corals more sensitive to the amount of algae they support.