Phosphates in Your Saltwater Aquarium

The Affect of Phosphates on Algae and Corals

Tropical fish swimming in an aquarium outside kitchen

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Phosphate, or PO4, is a compound of Phosphorus (P), one of the top 14 out of 70 trace elements found in natural seawater that are considered to be essential for saltwater aquarium and reef tank systems. The normal PO4 level in the ocean is 0.07 ppm (remember this number for future reference).

How Do Phosphates Affect the Saltwater Aquarium?

Phosphates are a primary nutrient source for many forms of algae, particularly green hair species, so when high concentrations are allowed to accumulate in an aquarium, it opens the door for aggressive algae blooms to occur.

Phosphates promote the proliferation of brown algae in the tissue of corals, masking the natural color pigments of the corals and causing the coral to turn brown. It also restricts the calcium carbonate uptake necessary to enable the coral skeleton to grow.

Where Do Phosphates Come From?

Phosphates are introduced into saltwater systems 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, foods 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 optimal phosphate level, especially for reef tanks, is an immeasurable one, or zero, with 0.05 ppm-mg/l being acceptable, and an upper level of 0.1 ppm cited as well. Having a good, reliable phosphate test kit is important. FINS recommends Hach and LaMotte brands as being good choices. Sailfert seems to have a good, precise phosphate test as well, but the Sea Chem scale measures too high.

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

  • Adding a limewater or kalkwasser solution has been associated with lowering phosphate levels. "In Notes From The Lab: Kalkwasser and Phosphates", a SeaScope Magazine Reprint, it states that, "One of the theories why calcium hydroxide might produce better results than calcium chloride has been that the high pH and high calcium in a saturated calcium hydroxide solution cause precipitation of phosphate from the freshwater, eliminating this algae fertilizer from the solution." Take a look at the results of the test they did using calcium hydroxide to see what the rate of reduction of phosphates were. It is interesting that, when allowing the mix to sit, after two days there was only a 50% drop in phosphates, but after five days there was a 90% drop. The conclusion is that the drop in phosphates, when using calcium hydroxide, is slow and if the mix is used right away, any phosphates remaining in the mix will be pumped into your tank.
  • 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 phosphate removing products, such as Sea Chem's PhosGuard, Kent Marine's Phosphate Sponge (Compare Prices) and Coralife's Phosphate Remover. 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 other top nitrate absorbing products 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 aquarium water is the "vodka method". 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 a protein skimmer.
Article Sources
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  1. Indicators: PhosphorusUnited States Environmental Protection Agency, 2020