Nitrates are present in nature as a result of the decomposition of organic matter. In a Koi pond nitrates will be produced when the fish waste is broken down from Ammonia, to Nitrite and then into Nitrates. Unless there is planting or water is changed, Nitrate levels can build up.
The Hanna Nitrate Tester Hi-93728 is an extremely easy to use Nitrate Low Range Meter and gives direct readout on the digital display in mg/L(ppm). To obtain a reading simply take a small sample of water in the glass cuvette supplied, add the reagent (not included), and then place the cuvette in the measuring chamber of the instrument.
With a normal 9V battery you will be able to get more than 300 measurements. The auto shut-off feature (after 10 minutes of non-use) guarantees a long battery life.
The Hanna Hi-83203 is also an accurate nitrate tester. It is a useful pond water meter that can also measure Ammonia, Nitrite, pH, dissolved oxygen, phosphates and Total Chlorine.