Nitrates form in pool water due to several different factors such as plant decay, contaminated well water, ground water, urine and sweat. A properly maintained pool has a range of nitrates from 10 to 25 parts per million or less. You can determine your pool's nitrate level using a testing kit from a pool supply retailer. If your pool has high levels of nitrates, it is at risk of an algae bloom. Fortunately, controlling nitrate levels is part of the standard pool maintenance process.
Establish the nitrate levels using a testing kit. Follow the manufacturer's directions.
Connect hoses to the intake and exit ports of a waste pump and insert the intake hose into the pool. If your pool has a built-in drainage system, you can forgo renting a waste pump to drain the water.
Drain approximately 15 percent of the water from your pool in accordance with local laws and building codes. In many cases, you cannot drain chlorinated water into a sewer drainage system or down the street.
Turn off the waste pump and remove the hose from the pool. If you have a built-in drainage system, turn off the pool pump.
Replace the water with the same water you normally use when you top off your pool. Generally, this is hose water.
Wait 24 hours and retest the nitrate levels in your pool with the test kit. If necessary, repeat the process. Again, nitrate levels must be below 25 ppm and preferably below 10 ppm.