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How to measure pond water turnover & filter flow rates

Updated July 19, 2017

The perennial problem that plagues anyone with a water feature is how to keep their pond water clear and clean. Water plants help tremendously when it comes to keeping the water clean and healthy. Photosynthesis removes the released carbon dioxide and reoxygenates the water with released oxygen. As long as you have only healthy water plants in your water feature, the environment can be easily maintained with minimal effort. However, if you have fish in your pond, the dynamics change dramatically. While water plants certainly help in maintaining a healthy pond, the addition of fish will necessitate using a filtration system–either mechanical or biological–to ensure clean, clear water. Pond water turnover rates and filter flow through rates become critical factors when using a filter. If the turnover and flow through rates are too low, the pond’s healthy environment will rapidly degrade.

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Determine your water feature’s volume by calculating the area of the pond then multiplying the area by 7.5 to get the volume in gallons:

Rectangle: Length x Width=Area

Oval: Measure from the pond centre to the most distant edge (A), then measure from the centre to the nearest edge (B). Multiply A x B = C then C x 3.14 = Area

Circle: Measure from the centre to the edge to get the radius (r). Multiply r x r x 3.14 = Area

Abstract and irregular: Multiply the maximum length by the maximum width = Area

Oblong: Break the space into two half-circles and one square. Calculate the area of the square using L x W and the area of the two half-circles by joining them and using r x r x 3.14. Add the two results together for the total area of the oblong.

Determine your water feature’s turnover rate by reviewing the gallons per hour (gph) rate of your pump with the filter attached compared to the total volume of your water feature, for example, a 1,000 gallon water feature needs a pump that can cycle through at least 500 gph to ensure the total volume is ‘turned over’ once every two hours, 12 times per 24 hours.

Determine the filter flow through rate by dividing the concentration of ammonia and nitrate by the optimum level for the water feature to get the ideal flow through rate to achieve that level; for example, the optimum level of ammonia and nitrate for Koi ponds is 0.0025 milligrams per litre (mg/L). Assume a production of ammonia and nitrate of 817 mg/day, you can find the ideal flow through rate with this equation: 817 mg/day divided by 0.0025 mg/L = 326,800 L/day or, 86,275 gallons/day = 3,535 gal/hr or about 60 gal/min


Maintain a supply of water treatment sufficient to treat the total volume of your water feature in case of contamination. Don’t skimp on gph when selecting a pump as ponds should have a turnover rate of at least once every two hours; optimum turnover is once every hour or 24 times per 24 hours. Contact your local water authority prior to introducing fish into your water feature and request a list of chemicals used to treat public water. Measure the pH of your water feature to determine the chemistry of the water prior to any intervention. Take note of the varying amounts of chlorine, chloramines, ammonia and nitrate as these chemicals can rapidly reach toxic levels that will kill fish. Treat the water, if necessary, to remove toxic levels of any harmful chemicals using commercial water treatments.


Chloramine (chlorine combined with ammonia) is often found in public water and will kill fish. You can detect its presence using a water feature pH kit. If found, it is necessary to treat the water with a chloramines removal agent prior to introducing fish.

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Things You'll Need

  • Calculator
  • Measuring tape

About the Author

A writer for more than 25 years, KT O'Neill has been a copywriter and editor for several UN agricultural organizations. She loves to take extremely technical documents and transform them into reader-friendly copy. She has two Bachelor of Arts degrees in linguistics and international relations, along with graduate work in international relations and finance.

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