GFO

Definition

GFO (Granular Ferric Oxide) is a phosphate-removing media used in reef-tank reactors. It binds inorganic phosphate (PO4) directly, reducing nuisance algae growth and freeing up dissolved phosphate that drives bryopsis, dinoflagellates, and cyanobacteria blooms.

Practical use in aquariums

Run GFO in a fluidized reactor at slow tumble (just enough to keep media moving). Replace every 4-6 weeks based on phosphate test results. Combine with carbon dosing (vodka/vinegar) for fastest reduction. Watch for too-rapid drops - sudden 0 ppm can shock corals adapted to higher phosphate.

How GFO fits the bigger picture

Understanding GFO matters because it's connected to broader husbandry decisions: equipment selection compounds: skimper + return + dosing all need to match each other and the tank size.

Browse the Fast Aquatics care library for full husbandry tutorials covering GFO in context.