How to lower Phosphates in a reef aquarium

Written by Adam Farrelly


In a previous blog, we discussed the causes and effects of high Nitrate in a saltwater aquarium. Phosphate is something that goes hand in hand with the latter, consisting primarily of organic and chemical waste and, whilst found in a much smaller concentration in saltwater, can have a significant effect on the aquarium.


In this blog we shall discuss the various types of phosphate found in saltwater, how the levels of said phosphates build up overtime, and how to control high Phosphates.


What is Phosphate?

Phosphates consist of compounds of PO4  groups (One atom of Phosphorus to 4 atoms of Oxygen) and are used by all life on earth (including in our own bodies) for protein synthesis. The two main types of Phosphate found in reef aquariums are Organic Phosphate (amino acids and proteins) and Inorganic Phosphates (Orthophosphates).


Common Sources of High Phosphate

These Include

  • Overfeeding (especially with pellets and coral foods)
  • Overstocking
  • Inadequate Filtration
  • Poor Maintenance

Consequences Of High Phosphate

  • Algae Blooms
  • Bacterial Blooms
  • Brown Corals 
  • Slow or nonexistent coral growth


Controlling Phosphate Levels and the Nitrate/Phosphate Ratio

The first thing you should remember when it comes to controlling Phosphate, you must control Nitrate as well, as the two go hand in hand. An imbalance of one can cause undesirable organisms to bloom (Dinoflagellates, Cyanobacteria etc.) due to their preference for one being much higher than the other. The ideal ratio of Nitrate to Phosphate levels in a reef aquarium is generally 16/1 (Nitrate levels are usually 16 times higher than Phosphate levels). Thankfully, the control methods for Phosphate are similar to those of Nitrate.


Phosphate Removal Methods 

As discussed in a previous blog about controlling Nitrates, the most obvious way one can control Phosphate levels is with regular water changes. Changing 10 to 20% of the aquarium's water volume per week is ideal for preventing a buildup of Phosphate in the aquarium and comes with additional benefits such as the removal of toxins. But as is the case again, locating the source of the elevated Phosphate levels will always be the best place to start.

Phosphates can be removed in a number of ways, but a good place to start is with a good Protein Skimmer such as the Bubble Magus Curve Series. These machines make use of a pump driven venturi to agitate water as it is sucked into the skimmer chamber, resulting in floating solids and settling as a fine foam at the top, perfect for removing any organic phosphates in the water.


As with Nitrates, Carbon Dosing with products such as Red Sea NOPOX will speed up the process and maintain a balanced ratio of Nitrate and Phosphate. Another Chemical method is with the use of Lanthanum Chloride (LaCl3) additives such as Quantum Phosphate Remover and ATM Agent Green. These will chemically bind phosphate in the water column, where it can be removed by a protein skimmer.

Possibly the most effective way to outright remove phosphate from the water is with a Granular Ferric Oxide media (GFO) such as ROWAPHOS, which can be combined with a reactor to further speed up the absorption process. This again chemically binds phosphate, this time to the media itself, and can rapidly strip phosphates from the water column.

With a sumped aquarium, macroalgae cultivation is also effective, as it is with Nitrates. Macroalgae such as Chaetomorpha can grow rapidly, and are a great way to prevent outbreaks of cyanobacteria and dinoflagellates as they essentially outcompete those species for nutrients. Be sure to periodically harvest excess macroalgae in order to prevent overgrowth and to physically remove nitrates from the system.


We at will always be happy to answer any questions regarding any thing discussed in this blog, as well as any questions regarding solutions to high nitrate. Feel free to contact us via the website or any of our social media links below.

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