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Ammonia, nitrite and nitrate: what they mean and what to do

These three numbers tell you whether your filter is keeping the water safe. In an established tank, ammonia and nitrite should both read zero. Nitrate is far milder but still needs managing.

Freshwater aquariums · Updated 8 October 2026

The nitrogen cycle in two minutes

Fish produce ammonia through their gills and waste. Uneaten food and dead plant matter add more as they rot. Beneficial bacteria living mostly in your filter media and on surfaces convert it in two steps:

1. Ammonia is turned into nitrite by one group of bacteria.
2. A second group turns nitrite into nitrate.

Nitrate then builds up until you remove it with water changes or fast-growing plants take it up. In a healthy, established tank, ammonia and nitrite sit at zero and nitrate drifts upward between water changes.

Quick reference

The ideal nitrate level depends on what you keep. Sensitive fish, shrimp and many planted tanks do better at the low end, while hardy community fish tolerate more. Check the needs of your own species. The 20 to 40 ppm figure is a common keepers' guideline, not a universal safety limit.

Ammonia

Any reading above zero in an established tank is a reason to investigate. How much it matters depends on what the number actually measures.

What your ammonia test measures

Most aquarium kits measure total ammonia, which is two forms added together: toxic ammonia (NH₃) and the much less harmful ammonium (NH₄⁺). Your kit's leaflet says which form it reports. The balance between the two shifts with pH and temperature: warm, alkaline water holds more of the toxic form, so the same total reading can mean very different levels of risk.

These are approximate equilibrium values, not safe limits, and the share rises by roughly a third for every 5 °C of warming. A test kit gives you the total and cannot tell you the NH₃ share, so treat any reading above zero as something to act on, and act sooner in warm or high-pH water.

If ammonia reads above zero

1. Test again to confirm, with a fresh sample and a clean tube. If fish are gasping or clearly in distress, do not wait for the result: start with steps 2 and 3, and test afterwards.
2. Do a partial water change using dechlorinated water at the same temperature. A quarter to a half is a sensible start. If the reading is clearly high and fish are distressed, a larger change is the right call, because it dilutes the ammonia directly.
3. Add aeration: point the filter outlet at the surface or add an air stone. Extra oxygen does not remove ammonia, but it helps stressed fish breathe while you fix the cause.
4. Skip feeding for a day and take out any uneaten food.
5. A water conditioner that claims to detoxify or bind ammonia may buy a little time. Products differ and the effect is not guaranteed, so it is no substitute for water changes and a working filter, and a test may still show the ammonia afterwards.
6. Look for the cause: a dead fish or snail, a clogged or switched-off filter, a recent filter clean, too many fish added at once, or medication that harmed the bacteria.
7. Test again after the change, and repeat it until the reading is down.

Nitrite

Nitrite is the middle step, so it shows up when the second group of bacteria has not caught up with the first. In an established tank it should read zero. A reading means the cycle has been disturbed or the tank is overloaded.

Nitrite harms fish differently from ammonia. It enters the blood and turns haemoglobin into a form that cannot carry oxygen, so affected fish can show signs of oxygen shortage, such as gasping, even when the water holds plenty of oxygen.

If nitrite reads above zero

• Do partial water changes, and repeat daily while the reading stays up. If fish are gasping, a larger change is justified.
• Add aeration. Extra oxygen does not remove nitrite, but it helps fish cope while the filter catches up.
• Feed lightly. Less food means less ammonia to process.
• Leave the filter media alone. Rinse it only in old tank water, never under the tap, since chlorine kills the bacteria.
• Check that the filter is running and the media is not clogged.

Nitrate

Nitrate is the end product, so it is the number you control with routine maintenance. It rises when you feed heavily, keep many fish, or go too long between water changes.

If nitrate is high

• Increase the size or frequency of your water changes.
• Test your tap water too. If it already holds nitrate, your changes cannot bring the tank below that number.
• Reduce feeding and clean out debris from the substrate and filter.
• Add fast-growing plants, which use nitrate as food.
• Look at stocking. A crowded tank makes nitrate faster than any routine can remove it.

When you are lowering nitrate as routine maintenance, bring it down gradually over several water changes rather than swapping nearly all the water in one go, which can shock fish with sudden changes in temperature and chemistry. An emergency is different: if ammonia or nitrite is high and fish are in distress, a large water change is the right call. In both cases, matching the temperature and treating the new water matter more than staying under a fixed percentage.

Read the trend, not just the number

One reading is a snapshot. What matters is direction: nitrate that climbs faster each week, or a brief ammonia spike every time you add fish. Testing at the same point in your routine, ideally just before a water change, shows the real peak. A log makes patterns visible long before fish show symptoms.

If you are unsure how to get a reliable reading, see how to read an aquarium water test.

This guide covers freshwater tanks. Saltwater and reef aquariums use different targets. If fish are gasping at the surface, do not wait for a test result: do a large water change with temperature-matched, treated water, add aeration, and then work out why.