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How to Test for Phosphorus (Phosphate) in Your Aquarium

Blue Devil

To keep your saltwater reef aquarium healthy, you want to maintain low phosphate levels. It's important to measure phosphates weekly in a reef tank because if the levels get too high (over .05 ppm) it can prevent coral calcification or cause excessive algae growth. (It's also a good idea to test the levels if you notice an excessive amount of algae in your tank.)  The easy-to-use Hanna Phosphorus Marine Ultra Low Range Checker HC - HI736 generates accurate, digital readouts in parts per billion (ppb) versus trying to judge traditional phosphate test kit color charts.

Check Out the Hanna Phosphorus ULR CheckerIn addition to being easy and accurate, the full line of Hanna Marine Checkers eliminate the hassle of always having to buy new test kits. When you run low on the supplied packets of HI736-25 reagent, just purchase a refill pack and you are back to testing in no time.

How to Use the Hanna Marine Phosphorus Ultra Low Range Checker HC - HI736

Video: How to Use the Marine Phosphorus Checker (3 minutes 30 seconds)


Note that the Hanna Marine Ultra Low Range Phosphorus Checker HC - HI736 gives readings in ppb.

  • To convert from ppb phosphorus to ppb phosphate, multiply by 3.066.
  • To convert the result to parts per million (ppm) phosphate, divide by 1,000.

Everything You Need to Start Testing

Our marine phosphorus checker is supplied as a complete test kit and retails for $49. The kit includes:

  • Hanna Phosphorus Marine Ultra Low Checker HC - HI736
  • Two sample cuvettes with caps
  • Phosphorus Ultra Low Range reagent starter kit
  • One AAA Battery
  • Water resistant case to store all testing items
  • The HI736 instruction manual and laminated quick reference guide. The reference guide includes the testing instructions and tips for accurate measurement. It’s the perfect companion to have while you are performing measurements.

How to test:

The Hanna Marine Phosphorus Ultra Low Range Checker is designed to make testing quick and easy.

  1. Press the button to power the meter on. When Add C1 appears on the screen, you are ready to begin testing.
  2. Fill a clean cuvette to the 10mL line with sample and fasten the cap.
  3. Using a microfiber cloth, wipe the cuvette to remove any oils or fingerprints from the glass. Then place the cuvette into the checker and close the lid. It's important to make sure that the lid is completely closed for an accurate reading.
  4. Press the button. Once the screen shows Add C2, remove the cuvette from the meter and add one packet of HI736-25 reagent.
  5. Fasten the cap, and shake gently for 2 minutes until the powder is completely dissolved. Then wipe the cuvette of any oils and fingerprints, and place back into the meter.
  6. Press and hold the button until the timer is displayed on the screen. The 3-minute countdown will begin. Once the timer reaches zero, the meter will automatically display the concentration of phosphorus in ppb.

Why Test for Phosphorus?

While phosphorus is essential for all life, it can be problematic for aquarium inhabitants – especially for coral, where excess phosphorus can prevent calcification of the coral from happening. The main source of phosphorus in reef aquariums is food, so care should be taken to avoid excess phosphates from feeding. Adding tap water to an aquarium can also lead to increased phosphate levels. Many municipalities have levels of phosphates in their water that are safe for human consumption, but can be detrimental to coral health. 

Additionally, excessive algae growth , a lack of protein skimming, poor water change schedule, and the use of tap water will all contribute to high phosphorous levels. Switching to reverse-osmosis deionized water (RO/DI) as a source for both topping off and making saltwater can help cut back on total phosphorus levels in the aquarium environment. Utilizing a refugium to grow beneficial macro-algae can also help export phosphates in a reef aquarium. Using filter medias and chemicals, like granular ferric oxide (GFO) or carbon dosing, can help bind or utilize phosphates and remove them from the aquarium. 

Although elevated phosphorus levels in an aquarium can be damaging, a complete absence of phosphorus is also not ideal. Phosphorus is essential for tissue growth and tanks that lack it completely can develop problems. The ideal level of phosphorus should be greater than 0 but less than 0.05ppm. Since so many factors are attributed to high phosphorus levels, every aquarist should be testing for them on a regular basis.

The Limits of Traditional Color-Change Test Kits

The ideal phosphorus level for reef aquariums is extremely low. Many other test kits on the market rely on a color change that needs to be judged by sight. These are notoriously difficult to use, especially in the lower ranges. The Hanna Marine Phosphorus Ultra Low Range Checker HC - HI736 bridges the gap between simple chemical test kits and professional instrumentation. The one-button design makes testing easier and more accurate by outputting a consistent digital reading every time.

Want to learn more about how accurate our Marine Checkers are? Check out the video below by our friends at Bulk Reef Supply, comapring our digital Checkers to a more expensive professional meter.

Video: Hanna Phosphate Checker Comparison by Bulk Reef Supply (10 minutes)

Learn More About Our Full Marine Line of Checkers

And Don't Forget Your Phosphorus Reagents!


Kevin Costa

Written by Kevin Costa

Kevin used to operate an aquarium maintenance company and worked at local fish stores before working at Hanna. He is a passionate aquarium hobbyist who enjoys reef keeping and planted aquariums. He graduated New York University and received a graduate certificate in aquaculture from the University of Florida specializing in fish health. Kevin may be reached at or 401-450-6051.

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