Before discussing how much GFO media you will need, it's important to understand that not all reef aquariums NEED phosphate removal media like GFO.  While GFO is probably the best option for the reduction of elevated phosphates in reef aquaria, it's not absolutely essential for all aquariums and doesn't need to be used at all times to maintain acceptable levels of phosphate. GFO should only be used when phosphate levels have risen beyond an acceptable level.

Once phosphates are returned to the normal range, it is best to evaluate your filtration and maintenance routine and make necessary changes to keep those phosphates within an acceptable range without the everyday use of GFO. Aquariums that require constant application of GFO to keep phosphates under control means the nutrient input is outpacing the filtration capacity of the aquarium. Most hobbyists keep GFO on hand at all times but only use the media on an as-needed basis. 

How Much GFO Do I Need?

There are three factors that come into play when calculating how much GFO media to use at one time. 

  • Water volume
  • Type of GFO
  • Total desired phosphate reduction

NOTE: DO NOT REDUCE PHOSPHATE LEVELS TOO QUICKLY to avoid stressing out your corals. No more than 0.5 ppm phosphate reduction in a 24-hour period. 

While you can always just use our phosphate media calculator, here is the correct dosage for the BRS brand Granular Ferric Oxide. Across the board, most GFO media is fairly similar and will use very similar dosages but nonetheless, always follow the manufacturer's instructions. 

Bulk Reef Supply Standard GFO

  • 1 tbsp per 4 gallons of aquarium water

Bulk Reef Supply High-Capacity GFO

  • 1/2 tbsp per 4 gallons of aquarium water

After the introduction of GFO, you should be testing your phosphate levels daily to see how much of a reduction you are achieving. GFO media should be changed out as soon as you notice phosphate levels stop falling and start to rise again. 

For particularly severe phosphate problems or systems with high phosphate input, you can gradually increase the dosage to a maximum of 2 tbsp per 4 gallons of aquarium water. Ideally, phosphate levels in a reef aquarium should be maintained at 0.05 ppm and you should only consider increasing the dosage if those optimal levels cannot be maintained. 

GFO Tips & Advice

  • Your very first batch of GFO will typically exhaust quickly, usually within 1-2 weeks, because of the initially high phosphate levels. Your second batch of GFO media should last anywhere from 4 - 8 weeks depending on your level of phosphate input.
  • GFO media is not reusable and should be removed as soon as it is exhausted or "full of phosphate" because it will not continue to remove phosphate thereafter. 
  • GFO is best used inside a fluidized media reactor for optimal contact time. 
  • Test phosphate levels daily to monitor GFO performance. We recommend the Hanna Instruments Low Range Phosphate Colorimeter
  • High-capacity GFO holds twice as much phosphate which means you can use half as much media to achieve the same level of phosphate reduction compared to standard GFO.
  • If you find that your aquarium requires constant use of GFO media to maintain acceptable levels of phosphate, it means your nutrient input is exceeding what your filtration can export. Feeding habits, stocking levels, and general filtration capacity can be evaluated to help reduce the reliance upon GFO media.
  • Reducing phosphate levels too quickly will stress out your aquarium. The drop in phosphate level should always be gradual when dealing with elevated phosphates.
  • Phosphate is not evil and is actually fundamental for the survival of corals and other organisms in your aquarium. The key is balance, keeping low levels of phosphate without allowing those levels to climb beyond acceptable levels. 
  • Elevated phosphate over time can fuel algae growth, impede the growth of stony corals, and cause an undesirable change in coral's coloration (brown out).