High nitrate levels in a fish tank could make fish sick or kill them. It can also cause a bad odor or cloud water. If you think your nitrate is high or you test the water and confirm it is, then you should lower the levels before fish begin dying off.
1. Feed your fish the correct amount. The number one reason for high nitrate levels in a tank is overfeeding. Make sure your fish finish their food in 2 minutes or less. You may want to feed your fish twice a day instead of dumping a large amount in once a day.
2. Decrease the number of fish in your tank. In general, you should have 1 gallon of water for each tropical fish and 3 gallons for each goldfish. More fish means more waste, which leads to nitrate buildup.
3. Vacuum the substrate. Waste collects in the rocks, so you need to use an aquarium vacuum to clean out the gravel. Also, clean the tank walls and decorations at this time.
4. Replace filter cartridges, which may be holding decaying food and waste in your tank, raising the nitrate levels. Increase the overall aeration by cleaning filters or adding additional pumps or hoses. This should lower your nitrate level.
5. Change the water after a test for high nitrate levels. Replace about half of the water and then retest. In general, you should replace about 20 percent of the water every 2 weeks to keep nitrate levels from building.
6. Add 50 mg of aquarium salt per gallon of water in your freshwater tank. The salt will not harm the fish and will be removed during water changes.
7. Test the water 24 hours after changing the water. If levels are still high, you should replace another 20 percent of the water and then retest the next day.
8. Continue small water changes each day until nitrate levels drop below 50 ppm. Anything over 60 ppm is considerably too high and may lead to a bloom that causes cloudy water.