GFO (Granular Ferric Oxide)
Antiphosphate resin based on iron oxide. The most widespread method for controlling phosphates in marine aquariums.
Detailed explanation
GFO (granular iron oxide) is an adsorbent resin that selectively removes phosphates (PO4) from water. It is the most used tool for controlling phosphates in the reef.
It works by binding phosphate ions to the surface of iron oxide. The adsorption capacity depends on the quality of the product: high quality GFOs (such as Rowaphos, PhosBan, Two Little Fishies Phosban) can remove phosphates to concentrations lower than 0.01 ppm.
It is used in two ways: in a bag in the sump with passive flow (less efficient but simple) or in a dedicated reactor with controlled flow (more efficient and precise). In the reactor the flow must be adjusted so that the GFO moves slightly (fluidified bed) without being violently shaken.
The GFO should be rinsed well before use to remove the red powder. The dust is not toxic but temporarily turns the water red and can clog the filter sponges.
The replacement must be done when the phosphates start to rise, typically every 4-8 weeks. Don't wait for the GFO to run out completely: by then the phosphates have already risen.
Excess GFO can strip phosphates to zero, starving coral zooxanthellae and causing bleaching. The goal in the reef is not zero phosphate but a low and stable level (0.02-0.08 ppm).
Alternatives to GFO include aluminium-based resins (less recommended due to the risk of aluminium release), lanthanum-based resins and biological methods (macroalgae, bacteria).