Your Saltwater Aquarium's Phosphates

Tropical fish swimming in an aquarium outside kitchen

Phosphate, or PO4, is a Phosphorus (P) molecule that is regarded vital for saltwater aquarium and reef tank systems. Phosphorus (P) is one of the top 14 trace elements present in natural seawater. The usual amount of PO4 in the ocean is 0.07 parts per million (remember this number for future reference).

How Do Phosphates Affect the Saltwater Aquarium?

Phosphates are a main nutrition source for many types of algae, especially green hair species, therefore allowing large quantities to collect in an aquarium can lead to severe algae blooms.

Phosphates encourage the growth of brown algae in coral tissue, concealing the coral's inherent color pigments and causing the coral to turn brown. It also prevents the coral skeleton from growing by limiting calcium carbonate intake.

Where Do Phosphates Come From?

Phosphates are introduced into in various ways, such as:

  • By using unfiltered tap water for making up sea salt mixes or as top-off water.
  • Through many common aquarium products used, such as some sea salt mixes (compare the levels), activated carbon, KH buffers, put into the tank, and many other sources. It is advised to pretest tap or any other freshwater source and new products used in your aquarium for phosphates as well as nitrates. Why add more of something that you don't need in your tank to cause a problem?

How Do You Reduce High Phosphate Concentrations?

The ideal phosphate level, especially for reef tanks, is an immeasurable one, or zero, with a lower limit of 0.05 ppm-mg/l and a higher limit of 0.1 ppm mentioned. It's critical to have a decent, dependable phosphate test kit. Hach and LaMotte are two brands that FINS recommends. Sailfert also appears to have a nice, precise phosphate test, but his Sea Chem scale readings are too high.

Here are solutions for reducing high phosphate concentrations in saltwater aquariums.

  • Regular maintenance care routines with water changes are always recommended for a healthy aquarium. This helps to control phosphates, DOCs, nitrates and many other unwanted chemicals elements and compounds in saltwater systems that contribute to water quality problems.
  • You can use commercially available , such as Sea Chem's PhosGuard, Kent Marine's Phosphate Sponge and Two Little Fishies Phosban. Luis Mercado conducted a test on phosphate and silicate removing compounds using these four products, which concluded comparatively good results from all four of them. Poly Filter material by Poly Bio Marine Inc. is highly recommended, along with as well.
  • Note that when using absorbing materials or compounds, they need to be changed or rejuvenated regularly, as once they become saturated they are exhausted. This means they can no longer absorb anything, and what you are trying to remove is just staying in your system.
  • Another simple, inexpensive method for extracting PO4 from your is the "". This method (the periodic addition of small amounts of vodka or ethanol to the aquarium) has been found to effective for isolating both nitrates (NO3) and phosphates (PO4) when used in conjunction with .


"Indicators: Phosphorus. United States Environmental Protection Agency.", "

Wang Lu et al. Corals and Their Microbiomes Are Differentially Affected by Exposure to Elevated Nutrients and a Natural Thermal Anomaly. Frontiers in Marine Science, vol. 5. 2018. doi:10.3389/fmars.2018.00101  " ;