GFO Calculator
Enter your aquarium details to calculate the recommended starting amount of GFO needed.
Start slow: GFO removes phosphate quickly and effectively. Begin with the recommended starting dose and monitor phosphate levels closely. If corals show negative reactions after 24 hours, reduce the amount by half and increase gradually over time.
Start Slow: Recommended Dosage
The recommended starting dose is 1 gram of GFO per gallon of total system water volume.
Rinse all new media before use and place it in a media bag, filter sock, canister filter, or fluidized reactor.
Monitor phosphate levels weekly. If phosphate remains above 0.15–0.25 ppm, replace or increase GFO as needed.
If after 24 hours you notice any negative coral reactions, reduce the amount by half.
Why Use GFO?
GFO, or granular ferric oxide, helps remove phosphate from aquarium water. Lower phosphate levels can help reduce nuisance algae and support a cleaner, more stable aquarium.
How This Calculator Works
This calculator multiplies your total system water volume by the recommended starting dosage of 1 gram per gallon. If liters are selected, the calculator automatically converts liters to gallons before calculating the dose.
Supported Products
- Reef Genetics GFO – High quality granular ferric oxide for effective phosphate removal.
- Reef Genetics High Capacity GFO – Higher capacity media for longer-lasting phosphate control.
- RowaPhos – Iron-based phosphate removal media designed to reduce phosphate and help control nuisance algae in reef aquariums.
Pro Tip
Start slow. GFO removes phosphate quickly and effectively. Always test your water and adjust gradually to avoid shocking your aquarium.
GFO calculators by brand & phosphate remover
RowaPhos calculator
RowaPhos is a premium iron-based (ferric hydroxide) phosphate remover prized for its high capacity and gentle action. Select RowaPhos in the calculator above and enter your system volume to get a conservative starting amount. As with any GFO, rinse it well, run it in a reactor or media bag, and reduce phosphate gradually.
Reef Genetics GFO calculator
Reef Genetics GFO and High Capacity GFO are granular ferric oxide media for reliable phosphate control. The high-capacity version binds more phosphate per gram and lasts longer between changes, making it a good choice for heavily fed or higher-nutrient systems.
Phosphate Rx vs. GFO
Phosphate Rx is a liquid lanthanum-chloride phosphate remover, not a granular media, so it works differently from GFO. It acts fast and is dosed in very small amounts with mechanical filtration running to catch the precipitate. Many reefers use GFO for steady day-to-day control and reserve a product like Phosphate Rx for occasional targeted reductions.
Frequently Asked Questions
How much GFO should I use per gallon?
A safe starting dose is about 1 gram of GFO per gallon of total system water volume. Rinse the media first, run it in a reactor or media bag, and retest phosphate after a week. If phosphate is still above 0.15 to 0.25 ppm, increase gradually. The GFO calculator above scales this dose to your exact volume for RowaPhos and Reef Genetics media.
How often should I replace GFO?
Replace GFO when phosphate starts climbing again, typically every 4 to 8 weeks depending on your bioload and phosphate levels. High-capacity GFO lasts longer. Test weekly so you change media based on your actual numbers rather than a fixed calendar.
Will GFO hurt my corals?
GFO is safe when used correctly, but dropping phosphate too fast can stress corals, especially SPS. Start with the recommended dose, watch your corals for 24 hours, and if you see a negative reaction cut the amount in half and ramp up slowly. Steady, gradual phosphate reduction is always safer than a crash.
