Nitrate is the end product of the nitrogen cycle, and only removal lowers it. Most guides aim to keep it under about 20 to 40 ppm. Change 30 to 50 per cent of the water to cut a high reading, then feed less, stock lightly and add fast-growing plants to keep it down.
Learn how to lower nitrate in a fish tank by taking it out, because nothing in the water breaks it down on its own. A partial water change is the fastest fix: a 30 per cent change cuts nitrate by about 30 per cent. Feeding less, cleaning a dirty filter and adding fast-growing plants stop it climbing back. Test first, though, because a badly shaken test kit can give a false reading.
This guide sits in our Ammonia & Nitrate guides. If the tank is still new, read the cycling timeline first, since ammonia and nitrite are the urgent numbers while it cycles.
What Nitrate Level Is Too High?
Nitrate is the last step of the nitrogen cycle: beneficial bacteria turn ammonia into nitrite, then into nitrate, which is far less toxic. It builds up in a closed tank because fish keep producing waste and nothing removes the end product except you and your plants.
| Reading (ppm) | What it means | What to do |
|---|---|---|
| 0 to 20 | Where most guides want a healthy freshwater tank | Keep your routine |
| 20 to 40 | Common action line; Seachem says not to let it build far above about 40 | Tighten feeding, plan a water change |
| 40 to 80 | Fish often cope, but sensitive species and fry may not | Change 30 to 50 per cent of the water and find the cause |
| 80 to 100 and up | Aquarium Co-Op puts its upper ceiling at 80 to 100 ppm | Change water in stages over a few days |
Sources disagree on the ceiling, from about 40 ppm up to 80 to 100 ppm. Treat 40 ppm as a sensible line to act on for a normal community tank. Shrimp, fry and planted tanks do better lower, and a planted tank can even run short of nitrate below about 20 ppm.
Test First, and Test Properly
A nitrate reading is only useful if the test was done right. With API-style liquid kits, the second nitrate bottle settles out, and if you don’t shake it hard the result can read falsely low. Shake it for 30 to 60 seconds, or longer if it has been sitting. Then time the tube exactly as the instructions say, since leaving it too long can push the colour higher. Our guide to testing fish tank water covers the whole routine.
Also test your tap water. The Australian Drinking Water Guidelines set a nitrate guideline of 50 mg/L, so a supply near that would add to your tank every time you top up. Bore and rainwater tanks are the ones to check.
How to Lower Nitrate in a Fish Tank: Step by Step
- Test the tank and the tap water. Write both numbers down so you can see what the changes achieve.
- Do a big partial water change. Siphon out 30 to 50 per cent of the water, vacuuming the gravel as you go. The maths is proportional: a 30 per cent change cuts nitrate by about 30 per cent, and 50 per cent halves it.
- Treat the new water. Add a conditioner that names chloramine, and match the temperature to the tank within a degree or two.
- Stage it if the reading is very high. Over 100 ppm, spread the change across several days rather than one big swap, so fish aren’t shocked.
- Retest after 24 hours and repeat if you’re still above your target.
- Then fix the source with the steps below, otherwise the number climbs back within weeks.
Water changes can only ever approach zero. If your reading is 80 ppm and you change half the water twice, you land near 20 ppm, not zero. That is normal and fine.
Keep It Down: Feeding, Filter and Stocking
Nitrate comes from waste, so cut the waste. Feed only what your fish finish in about two minutes. Vacuum debris out of the gravel rather than letting it rot. A dirty filter can act as a ‘nitrate factory’, so rinse the media in old tank water when flow drops, never under the tap.
Stocking matters most of all. Too many fish for the volume will out-produce any fix. The simple answer is fewer fish, a bigger tank, or both. The rule of thumb about one inch of fish per gallon is too crude to trust, so judge by species and filtration instead.
Do Live Plants Lower Nitrate?
Yes, but with limits. Plants use nitrate as fertiliser, and fast growers such as hornwort and water sprite absorb it quickest. The catch is scale: the plant mass needed to make a real dent in a heavily stocked tank is large, and sources agree plants alone rarely fix a big bioload. Check the law before you buy: some aquatic plants are restricted, and hornwort is a declared weed in Tasmania, where its sale is prohibited. Rules differ by state and change, so ask your state biosecurity department or a local aquarium shop. Use plants as a steady brake alongside water changes. Trim fast growers and remove the cuttings, or the nitrate they took up returns as they decay. See our beginner plant picks for easy starters.
What About Nitrate Removers and Denitrifying Media?
Products exist, including denitrifying filter media and nitrate-absorbing pouches. Effectiveness in freshwater varies widely and evidence is mostly manufacturer and forum claims, so treat them as a supplement, not a replacement for water changes. Anaerobic denitrification works when bacteria in low-oxygen zones turn nitrate into nitrogen gas, but it needs slow flow through the media to work.
Common Mistakes
- Changing 100 per cent of the water. It shocks fish and wipes out the stable conditions the tank needs.
- Chasing zero. A reading of 10 to 20 ppm is fine, and planted tanks need some nitrate.
- Trusting one test. Check the shaking and timing before you panic.
- Treating the number, not the cause. Overfeeding and overstocking keep refilling it.
- Scrubbing the filter under the tap. Chlorine or chloramine can kill the bacteria that keep ammonia at zero.
When to Get Help
If nitrate stays above 40 ppm after several changes, or fish are gasping, clamped or lethargic, look wider than nitrate. Check ammonia and nitrite first. Those are the toxic ones, and they call for a different response. For rhythm and routine, see how often to change water.
Related Guides
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Frequently Asked Questions
How to lower nitrate in a fish tank fast?
Change 30 to 50 per cent of the water with treated, temperature-matched water. A 50 per cent change roughly halves the reading. Retest after 24 hours and repeat if needed.
How do I lower nitrates in a fish tank naturally?
Feed less, stock lighter and add fast-growing plants such as hornwort or water sprite. These slow the build-up, but for a stocked tank they work best beside regular partial water changes.
What is a safe nitrate level in a fish tank?
Most guides aim for under 20 to 40 ppm. Some sources tolerate up to 80 to 100 ppm, though shrimp, fry and sensitive species do better lower. Check your species’ needs.
Why is my nitrate high after a water change?
Either your tap water carries nitrate or the test was done wrongly. Test the tap water and re-shake the second nitrate bottle for 30 to 60 seconds before retesting.
Can I get nitrate to zero?
Not by water changes alone, since each change only removes a percentage. Nor should you need to. A low, steady reading is the goal.
