Excess phosphates are a problem in saltwater aquariums and especially reef systems…


They are the phantom menace for corals; invisible compounds that can retard coral growth and even kill!

If you want your corals thriving you need to get rid of phosphates!

If you want your corals thriving you need to get rid of phosphates

Phosphates as I’m sure you know are a nutrient in saltwater systems that come  from biological waste products and other substances we put into our water.

Phosphates negatively  affect the plants and animals in your tank:

1. Phosphates reduce water quality and promote pest algae growth.

2. Excessive phosphates inhibit calcification in corals and coraline algae so inhibit their growth.

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.

Where do they come from?

Phosphates are expelled from all organisms  (metabolic waste), they also are present in:

  • Many foods (especially frozen foods) contain phosphates as preservatives.
  • Untreated water sources (you should be using RO or DI water for a reef tank).
  • Substrates including crushed coral, dead reef rock (base rock) and other coral based rock can have high levels of precipitated phosphates.
  • Salt mixture (check the label).
  •  Additives (also check the label).
  • and even chemical media (activated carbon).

The harmful phosphates we want to minimise are in the form of precipitated (solid) or dissolved inorganic orthophosphates, which are detected by standard chemical test kits and are released into the water from one or more of the sources I mentioned.

saltwater aquarium corals and phosphate

Corals really dont like too much phosphates. (Photo credit: beautifulnatureblog.blogspot.com)

6 ways to get rid of phosphates:

Phosphate Test Kit

If you don’t know why you have high phosphate: test everything that goes into your tank!

So, now that your test kits reveal you have phosphate levels that are too high what do you do?

 1. 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. Macro-algae will not be a sufficient control if rock is the cause of your phosphates.

2. 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.

3. 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. Remember to clean out the skimmate once or twice a week.

4. 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. Remember everything you put into your water stays there in one form or another, so only put in what your tank really needs.

5. 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.

6. 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. Media needs to be washed every few days to remove the bacteria that will build up and block the phosphate absorption. The product I recommend is Phosban by Two Little Fishies.

So now you know what phosphate is, what it does, where it comes from and some of the most effective ways of getting rid of it (for me:  filtered water, water changes and macro-algae are the most successful ways).

Andrej Brummer
Saltwater Aquarium Advice Scientist




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