Ammonia, Nitrite, Nitrate: Reading Your Water Test Kit
Your test kit is the only way to see the invisible chemistry that keeps fish alive. This guide explains what ammonia, nitrite, and nitrate each mean, the target numbers to aim for, how to read the colours accurately, and exactly what to do when a reading comes back wrong.
Quick answer
In a healthy, cycled tank ammonia and nitrite should both read 0 ppm, and nitrate should sit below about 20-40 ppm. Ammonia and nitrite are acutely toxic; nitrate is the far milder end-product you dilute with water changes. Test weekly, and any time fish look unwell.
Your test kit is the only way to see the invisible chemistry that keeps fish alive.
What each number actually means
Fish constantly excrete ammonia through their gills and waste. In a cycled tank, one group of bacteria converts ammonia into nitrite, and a second group converts nitrite into far less harmful nitrate. This is the nitrogen cycle. When you test, you are checking that this bacterial chain is keeping up with the amount of waste your fish produce.
Ammonia and nitrite are the dangerous readings. Both damage gills and blood at low concentrations and can kill within a day at high ones. Nitrate is the comparatively safe end-product, but it still needs to be kept down because chronically high nitrate stresses fish and fuels algae.
How to read the kit accurately
Use a liquid reagent kit rather than strips where you can — strips drift badly with humidity, which matters in damp subtropical flats.
Read results against a white background in natural light for the truest colour match.
Hold the tube against the white section of the colour card in natural daylight, not under warm indoor bulbs that skew the colour. Read at the recommended time; some tests, especially nitrate, keep developing and will read falsely high if you wait too long. Shake nitrate reagent bottles hard, as the manufacturer instructs, or you will get low false readings.
Ammonia and nitrite should read zero in a healthy, cycled tank; only nitrate rises over time.
Target ranges at a glance
Aim for ammonia 0, nitrite 0, and nitrate under 20-40 ppm. During a new tank's cycle you will see ammonia rise then fall, nitrite rise then fall, and finally nitrate appear — that sequence is how you know the tank is maturing.
Quick FAQs
How often should I test? Weekly for a settled tank, and daily during cycling or after any problem.
Are test strips good enough? For a rough screen, yes, but confirm anything worrying with a liquid kit, which is more accurate.
Why is my ammonia never zero with a new tank? Because the bacteria are not established yet. That is normal during cycling; keep up water changes until it reads 0.
Do reagent kits expire? Yes — check the dates. Old reagents, especially nitrate, give unreliable results and should be replaced.