GFO and Phosphate Removers: Dosing, Placement and When to Change Them

Phosphate is one of those parameters that quietly decides whether your reef looks sharp or hazy. Too much and you get hair algae carpets and dull, brownish coral tissue. Too little and some SPS colonies bleach out or stall growth. Granular ferric oxide, usually shortened to GFO, and other iron or aluminium based media are the standard tools for keeping phosphate in a useful middle ground without swinging it to zero.

What does GFO actually remove, and why does coral colour change?

GFO and similar media bind orthophosphate (PO4) onto iron oxide or aluminium oxide granules through adsorption. As phosphate drops, nuisance algae that rely on it slow down and often recede. Corals respond too, but not always the way people expect. Many SPS colonies pale or lose fluorescence intensity when phosphate is pushed too low alongside very low nitrate, because zooxanthellae density drops. The goal with products like Seachem PhosGuard or a dedicated GFO 0.04 grade media is gradual, moderate reduction, not scrubbing the tank to undetectable levels.

How much GFO do I actually need?

Dosage is based on tank volume and starting phosphate reading, not guesswork. As a rough starting point for standard GFO:

• Under 200 litres: 50 to 100ml of media, tested weekly
• 200 to 500 litres: 100 to 250ml, split across a reactor if flow is high
• Over 500 litres: scale proportionally, often across two smaller reactors rather than one large one, for even flow distribution

Seachem PhosGuard is an aluminium oxide/hydroxide blend and is generally dosed slightly differently to iron based GFO, so check the specific product guide rather than assuming a 1:1 swap. Triton's AL99 phosphate remover is another aluminium based option some Brisbane reefers prefer for its slower, steadier release profile, useful in tanks where phosphate has previously crashed too hard too fast.

Reactor or media bag: which placement is better?

A dedicated media reactor, fed by a small pump and plumbed for slow, even flow through the granules, gives the most consistent contact time and is the better long term option for anyone dosing GFO regularly. Media bags sitting in a sump or hang on filter compartment work too, especially for smaller tanks or as a stopgap, but flow through a bag is uneven, granules near the outer edge get exhausted while the centre barely gets used. If you are running a bag, squeeze it gently and rotate its position weekly so all the media sees water movement. A reactor also lets you dial back flow rate, which matters because GFO run too fast can strip phosphate too quickly and shock corals.

How often should GFO be changed?

Change GFO before it is fully exhausted, not after. Most iron oxide media start losing binding capacity noticeably after two to four weeks of active use, faster in heavily stocked or overfed systems. A simple rule: if your phosphate test starts creeping back up despite the reactor running, the media is spent. Running old, saturated GFO does nothing and can even leach phosphate back under some conditions. Test phosphate weekly with a reliable kit, several accurate options sit in our test and monitoring hardware range, and treat the reading as your change trigger rather than a fixed calendar date.

Practical tips for stable results

• Rinse new GFO under tap water before use to remove fine dust that clouds the tank
• Never run GFO alongside a phosphate dosing product at the same time unless you are actively trying to hold a specific target
• Pair phosphate control with good mechanical filtration; a fresh filter sock catches detritus before it breaks down into more phosphate, easing the load on your media
• Reduce GFO dose or pause it if SPS colour starts fading unusually fast, this often means phosphate has dropped too far too quickly

GFO and phosphate removers from Seachem, generic GFO grades and Triton's AL99 are all things we carry from time to time as part of a broader filtration range, alongside mechanical media like filter socks that support the same goal. Because stock on specific media types shifts with supplier availability, check out our stock list for what is currently on the shelf before planning a reactor refill or a full media swap.