Phosphate is what is limiting in our reefs. If you control the phosphate (which inhibits coral growth) to the levels found in the ocean, your corals wil thrive. We can easily measure inorganic phosphate in our tanks.... but, most of the phosphate is locked up organically...ie...detritus,plants corals,fish,bacteria...etc. When any of these die (and bacteria dies and regenerates every 12 hours), there is a large amount of "inorganic" phosphate that is released back into the water column for all to feast on (bacteria,algaes, etc.). It is this reason that your water can test zero for phosphates, but, you still have a hair algae problem.... the huge amounts of phosphate are just temporarily "locked" up. Now this is where a phosphate reactor comes into play....the reactor is always locking up the phosphate as are the algaes...only the reactor takes it out of the system permanetly. When phosphate enters the system....sometimes the algae will get it and sometimes the reactor will get it...but, in the end, the reactor wins. Refugiums operate on a similar basis (harvest the macro and you remove phosphate)...but, it would take a huge refugium to be as efficient as the media.
The only phosphate reactor media that should be used is an iron based media. There are currently two on the market...Rowaphos and Phosban... I use Rowaphos. The iron based medias are most efficiently used in a fluidized reactor. Here's mine... it's the one on the right.