The short answer
For a typical freshwater community tank, keep nitrate below about 20–40 ppm. It’s the end product of your filter breaking down fish waste — harmless at low levels, but stressful and algae-fuelling when it climbs. There’s no single magic number: aim for under 20 ppm if you can, treat 40 ppm as a ceiling for most fish, and act whenever a test shows it creeping up.
Where nitrate comes from
Fish waste becomes ammonia, your filter bacteria convert that to nitrite, then to nitrate. The catch is that the biological cycle stops there — nothing in a normal filter removes nitrate. It simply accumulates between water changes. Understanding this loop is the whole point of learning how to cycle an aquarium.
Why high nitrate matters
Chronically high nitrate leaves fish listless, suppresses growth in young fish, harms sensitive species and shrimp, and feeds algae blooms including green water. It’s rarely an instant killer, but it’s a reliable marker that waste is outpacing your maintenance.
How to keep it low
- Water changes are the primary tool — a weekly 25–30% change exports nitrate directly. See how to do a water change.
- Live plants absorb nitrate as they grow; fast growers make the biggest dent.
- Feed less and stock sensibly — less input means less nitrate.
- For stubborn cases, read how to lower nitrates.
Test to know your number
You can’t manage what you don’t measure. A liquid test kit gives a real number rather than a guess, and checking it before and after water changes tells you whether your routine is keeping up. Browse the water testing hub for how the readings fit together.