From Shirlie Sharpe,
Your Guide to Freshwater Aquariums.
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Phosphates (PO4) are present in every aquarium, even though many aquarium owners aren’t aware of it. If the aquarium is not properly maintained, the phosphate levels will rise and contribute to algae growth.

Effect on Fish
Fortunately phosphates do not directly harm your fish, even at high levels. However, the algae blooms that result from elevated phosphates can ultimately cause problems for the aquarium inhabitants. For instance, green water can deplete the oxygen, which in turn can harm the fish.

Phosphates and Algae
There is no question that phosphate promotes algae growth.

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
  
• water itself
  

Desired Level
Phosphates are present in both organic and inorganic forms. Test kits are only able to 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 to start. At 2 to3 ppm, algae overgrowth is likely to occur. Ideal phosphate levels are 0.05 pmm, or less.

Reducing Nitrates
The best way to reduce phosphate in your aquarium is to never let it get high in the first place. However, if your phosphates are already high, take immediate steps to reduce it by taking the following steps.

• Water Change – Large water changes will help bring phosphates down quickly, but if the underlying sources are still there, it will only be temporary. Until all causes are cured, continue to perform frequent large water changes to keep phosphate levels manageable.
  
  
• Phosphate Absorber – Phosphate absorbing media is very effective. It can be added to virtually any filter, and will quickly begin removing excess phosphate.
  
  
• Tank Cleaning – Scrape 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, and then clean the filter.
  

Keeping Phosphate Low
Once you bring the phosphate level down, make sure it stays low. Here are some ways to avoid soaring phosphate levels.

• Feed Sparingly – The number one source of phosphate in the 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 Changes – Frequent water changes will help keep phosphate levels from rising. Change ten to 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 Media – Carbon 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 aquaria, 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 – Cleaning debris from the filter regularly 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. If they aren’t absolutely needed, don’t use them. When you do use them, research the product and choose the one that contains the least amount of phosphate.