Good quality aquarium water means consistently low levels of nutrients…


Phosphates, along with nitrates and silicates are the saltwater aquarium “big 3” of chemical nutrients you don’t want accumulating unchecked in your saltwater tank.

In low levels phosphates will harm corals and promote pest algae growth and in high levels they can kill corals!

Lately a lot of people have been contacting me asking “how can I get rid of phosphates“, so I thought it was about time I write an article on this issue.

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What are phosphates and where do they come from?


  • Phosphates are chemical compounds containing phosphorus and oxygen ions (PO4) that are present in almost all living organisms
  • They are expelled primarily as biological waste from your marine pets
  • they also are present in many foods, untreated water sources, substrates, additives and even chemical media you may put into your tank
  • the harmful phosphates we want to minimize are in the form of precipitated (solid) or dissolved inorganic orthophosphates
  • inorganic orthophosphates are detected by standard chemical test kits
reucing phosphate

Fish don’t mind phosphates but your corals sure do!


How much phosphates are really too much?


  • We actually need tiny amounts of phosphates (0.005 ppm) for good healthy coral growth and tissue regeneration
  • A natural coral reef contains 0.07 ppm of phosphate, this trace level prevents corals bleaching because zooxanthellae in coral tissues need a little to survive
  • More than 0.1 ppm causes problems in saltwater systems with sensitive corals like SPS corals
  • more than 0.25 ppm will cause problems for more hardy corals such as soft corals
  • High phosphate levels in saltwater aquariums do not affect fish at all, this means phosphates are primarily a problem in reef aquariums.

If your fish are doing great and your corals are all suffering it is a sure sign to check your phosphate levels!

how to reduce phosphates

Too many fish or a just a few large active fish in smaller tanks can produce enough phosphates to harm more sensitive corals unless you are onto it with your water testing and nutrient reduction strategies (image credit: iheartpandas)

Phosphates negatively affect your aquarium by acting as fertilizer to pest algae species and promoting their growth which can lead to outbreaks.

Often your test results will show phosphate levels are within acceptable levels – because the algae is using the high phosphate concentration up as fast as it is added to the water! Don’t be fooled…

Phosphates also inhibit coral growth by blocking calcium uptake and thus skeleton growth, this is especially problematic for hard corals and reef tank owners.

Phosphates can also discolor corals by encouraging growth of brown algae cells in corals, which can eventually turn the entire coral brown!

reef tank phosphates

If growth of your corals is slowing or stopped entirely and they appear to be losing color, too much phosphates is probably the problem (image credit: Glenn Marsch)


Top sources of phosphates in your aquarium


  1. Untreated water sources: If you are not using a reverse osmosis (RO) and/or deionised (DI) water treatment filter and you own a reef aquarium you should ask yourself why? Top off water straight from the tap is a major source of introduction for phosphates.
  1. Salt mixture: Does your brand of salt mixture contain phosphates? Have you checked? You should…
  1. Using certain types of rock in your tank: Unfortunately many types of rock (not live rock fortunately!) such as dead reef rock (base rock) and coral based rock/crushed coral used in saltwater aquariums can have high levels of precipitated phosphates and act as slow-release phosphate reservoirs. Phosphates contained in these usually slowly dissolve into the water over time. Crush a tiny bit of rock up in water and test it if you suspect your rock is to blame.
  1. Frozen foods and feeding: frozen foods especially have high levels of phosphates (which as added to preserve the food), flake foods contain some. All foods should be checked for phosphate content on the element make-up list where possible. Frozen meaty foods should be thawed and the liquid discarded seeing as it contains a high concentration of phosphates.
  1. Additives: Many additives that proclaim to be necessary for a healthy saltwater aquarium contain high levels of phosphates; check the label first! – Most of the benefit of additives can be gained using regular partial water changes to replenish important ions like iodine.
  1. Activated Carbon: There has been a lot of attention over the years on various activated carbon brands containing phosphate and so becoming a phosphate source in your tank. Search online for info on your brand.
poor aquarium water quality

Macro-algae is the perfect biological control for phosphates and nitrates


The best ways to get rid of phosphates


  • Use Macroalgae: Macroalgae is the perfect biological control for phosphates (and nitrates); it uses them up effectively and outcompetes pest microalgae species. As the macro-algae grows and locks up the phosphates from the water it can be harvested, thus getting rid of it for good
  •  Understock and underfeed: As heavy biomass and large feedings are a notorious source of phosphates in saltwater tanks, simply dial both of these back a few notches and kick back and watch as your pollution levels become much easier to control
  • Phosphate absorbing/binding media: An easy way to mop up those pesky phosphates, most work exactly as advertised and are best applied in a small mesh bag somewhere in high current flow, make sure you follow the replacement instructions and don’t over do it, because all living creatures need a small amount of phosphates to thrive
How to remove phosphate

In my humble opinion PhosBan by Two Little Fishies is the best phosphate removing/binding media on the market

  •  Protein skimming: A wonderful way of removing organic waste from your system before it throws your water quality out. Protein skimmers are mostly effective at reducing nitrates but they do help with phosphates to a smaller degree.
  •  Effectively reducing phosphates at the source: with a bit of testing you can establish exactly where your phosphates are coming from, eliminate them here for a long-term solution
  •  Regular partial water changes: the single most effective way of reducing phosphates (in the short term). I recommend changing at least 5% twice a week if you have a phosphate problem


Personally, I use a combination of regular partial water changes (10% per week), filtered water and macro-algae to keep my phosphates as low as possible.

Have any tips of your own? Comment below and share your experience with us :)

Hopefully now you know a little more about phosphates, where they come from and how to get rid of them!

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Saltwater aquarium advice