Phosphates in the Aquarium

The Effect On Fish, Aquatic Plants and Algae

Reef Tank Maintenance
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Phosphates (PO4) are present in every aquarium, even though many aquarium owners aren’t aware that they're there. If the aquarium is not properly maintained, the phosphate levels will continually rise and contribute to algae growth. Testing for phosphates and learning about the sources of phosphate in your aquarium water will help you combat their effects.

The Effect of Phosphates 

Fortunately, phosphates do not directly harm your fish, even when they're at high levels. The algae blooms that result from elevated phosphates can ultimately cause problems for the aquarium inhabitants, however. Green water can deplete the oxygen, which in turn can harm the fish.

Where Do Phosphates Come From?

Phosphates naturally occur as wastes are broken down within the aquarium. In addition to being internally produced, phosphates can enter the aquarium from external sources. Everything from food to the chemicals used to buffer the water to the water itself can contain significant amounts of phosphate. Phosphate sources include:

  • Uneaten food
  • Plant decay
  • Dying algae
  • Fish feces
  • Dead fish
  • Carbon filter media
  • Aquarium salts
  • pH buffers
  • kH buffers
  • The water itself

Desired Level 

Phosphates are present in both organic and inorganic forms. Test kits can only test for inorganic phosphate, so keep in mind that you are only testing a portion of the total phosphate in your aquarium.

When test results show levels of 1.0 ppm or 1.0 mg/L, the conditions become favorable for algae growth. At 2 to 3 ppm, algae overgrowth is likely to occur. Ideal phosphate levels are 0.05 ppm or less.

Reducing Phosphate 

The best way to reduce phosphate in your aquarium is to never let it get high in the first place. If your phosphates are already too high, however, you can reduce it by taking the following steps.

  • Water Change: Large water changes will help bring phosphates down quickly, but the fix will be temporary if the underlying sources are still there. Continue to perform frequent large water changes to keep phosphate levels manageable until all causes are fixed.
  • Tank CleaningScrape the inside of the glass. Remove the rocks and other decorations and scrub them well. Let everything settle a bit, then give the substrate a good vacuuming. Wait a few days to give things a chance to stabilize, then clean the filter.
  • Phosphate Absorber: Phosphate absorbing media is very effective. It can be added to virtually any filter. Note: Using chemicals should generally be your last resort.

Keeping Phosphate Low

When you've brought the phosphate level down, make sure it stays low. Here are some ways to avoid soaring phosphate levels.

  • Feed Sparingly: The #1 source of phosphate in an aquarium is flake food. Cut back on the frequency and amount of food. Just a pinch once a day is sufficient for most adult fish. Remove any uneaten food promptly.
  • Change Food: Phosphate is used as a preservative in flake foods. All brands are not created equal, so do your research and choose those brands that have lower phosphate levels.
  • Water Source: Test your water source. It is not unusual for tap water to contain 1 ppm of phosphate. If the level is high, seek an alternate source for your aquarium water.
  • Water ChangesFrequent water changes will help keep phosphate levels from rising. Change 10 to 15 fifteen percent weekly, using a low phosphate water source.
  • Tank Maintenance: Keeping the tank free of debris will help avoid phosphate buildup. Vacuum the bottom frequently to remove uneaten food, plant decay, and fish waste.
  • Filter MediaCarbon is a good filter media, but it can add phosphate to the water column so choose carefully. Some carbon media, such as those for saltwater aquariums, is formulated specifically to not leach phosphates into the water. Others combine carbon media with phosphate absorbers so you get the best of both.
  • Filter Cleaning: Regularly cleaning debris from the filter will help reduce the sources of phosphates.
  • Water Treatments: Buffers that condition the water, alter or stabilize the pH, add trace elements or change the hardness often contain phosphates. Don't use them if they aren’t absolutely needed. If you must use them, research the product and choose one that contains the least amount of phosphate.