Phosphate Remover (GFO): Controlling Phosphate
How granular ferric oxide binds phosphate in a media reactor to keep reef and freshwater phosphate low and starve nuisance algae.
Overview
A phosphate remover based on granular ferric oxide (GFO) is media run in a reactor to bind phosphate from saltwater and freshwater systems. It is widely used to keep reef phosphate low and to starve nuisance algae of a key nutrient.
Why phosphate matters
Phosphate is the most common environmental form of phosphorus and the only form that plants and algae can use directly. Elevated phosphate fuels excessive plant and algal growth; in freshwater bodies this drives algal blooms, and as dead algae decompose, bacteria deplete dissolved oxygen, a process known as eutrophication.
How GFO works
GFO is an iron oxide that removes phosphate through a binding reaction in which phosphate adsorbs to its surface. Phosphates readily adsorb to iron (oxyhydr)oxide surfaces in nature, and GFO applies the same chemistry in the aquarium. It has a high affinity for phosphate and silicate and rapidly removes ortho-phosphate.
Running it in a reactor
GFO is commonly run in a media reactor or filter sock where water is passed through the granules. Some keepers run a reactor continuously; others run it intermittently, switching it on when phosphate rises and off after a period, to keep phosphate within a target range.
Side effects
GFO can drop alkalinity when first added to a system. Adsorption is also not strictly permanent: phosphate bound to GFO surfaces can re-enter the water column by exchange, so the medium sequesters phosphate temporarily and should be replaced as it becomes exhausted.
Practical notes
- Run GFO in a reactor or filter sock with controlled flow
- Introduce it gradually to limit a sudden alkalinity drop
- Replace exhausted media so bound phosphate does not return
- Monitor phosphate to time replacement and reactor cycles