What your nitrate reading means
Good0 to 20 ppm
A good range for a community tank.
Watch20 to 40 ppm
Creeping up: change a little more water each week, or change it more often.
Act today40 to 80 ppm
High: bring it down with larger or more frequent water changes over the next week.
Act todayOver 80 ppm
Very high: bring it down over several water changes, not one huge change.
For a tank with fish. The same bands run in the test reading checker, which reads all your results together.
Where it comes from
Nitrate is the end of the nitrogen cycle: filter bacteria turn ammonia into nitrite and nitrite into nitrate. Little removes it in a closed tank, so it climbs between water changes. The University of Florida notes it can pass 250 ppm in systems with little or no water exchange. Plants, floating plants and algae take some up as fertilizer.
How harmful is it
Nitrate was long thought harmless to fish up to about 200 ppm, but more recent research suggests it does more harm than that, according to the University of Florida. High nitrate also feeds algae. Sensitive fish, fry and shrimp do best at the low end; 20 and 40 ppm are the marks most keepers use.
Bringing it down
Each water change takes out its own share: changing 25 percent of the water cuts nitrate by a quarter, so 40 ppm becomes 30 ppm, as long as your tap water has none. Two 25 percent changes in a row take 40 ppm down to about 23. Other help: feed less, vacuum part of the substrate at each change, rinse filter sponges in tank water, and grow fast plants or floating plants. Work out the water change · Floating plants
When to go slowly
Fish in an old tank with very high nitrate, low KH and a low pH have adapted to it. A sudden large water change can swing the pH and shock them. Bring the nitrate down with smaller changes every day or two, testing pH as you go. Aquarium pH
Nitrate in tap water
If water changes do not lower your reading, test the tap. If it carries nitrate, the tank can never read lower than the tap water, and plants, or water mixed with RO or distilled water, are the way down. Check all your readings together