You've shaken the bottle, waited five minutes and now you're staring at a vial of coloured water next to a printed chart wondering what it actually means for your fish. This guide explains what every colour in your aquarium water test kit means — and more importantly, what you should do about it.
Ammonia (NH3)
Ammonia is produced by fish waste, uneaten food and decaying plant matter. It is the most acutely toxic parameter in freshwater aquariums — even small amounts cause gill damage and stress.
| Colour | Reading | Safety |
|---|---|---|
| Bright yellow | 0 ppm | Safe ✓ |
| Yellow-green | 0.25 ppm | Monitor closely |
| Olive yellow | 0.5 ppm | Attention needed |
| Olive green | 1 ppm | Dangerous — act now |
| Dark green | 2–8 ppm | Emergency |
Emergency action at 2ppm+: Do an immediate 75–80% water change. Remove all uneaten food. Consider an ammonia detoxifier such as Seachem Prime. Retest after the water change.
Nitrite (NO2)
Nitrite is produced by beneficial bacteria breaking down ammonia. In a fully cycled tank, nitrite should be converted to nitrate almost immediately. Any nitrite above zero means your biological filter is not keeping up.
| Colour | Reading | Safety |
|---|---|---|
| Pale lavender/light blue | 0 ppm | Safe ✓ |
| Lavender | 0.25 ppm | Investigate |
| Purple | 0.5–1 ppm | Dangerous |
| Deep purple | 2–5 ppm | Emergency |
Nitrite binds to haemoglobin in fish blood — the same mechanism as carbon monoxide poisoning in humans — reducing oxygen-carrying capacity. Fish may appear at the surface gasping. Do a 30–50% water change immediately.
Nitrate (NO3)
Nitrate is the end product of the nitrogen cycle — the relatively safe form that ammonia and nitrite are eventually converted to. Fish can tolerate moderate nitrate levels, but high chronic nitrate causes long-term health problems.
| Colour | Reading | Safety |
|---|---|---|
| Very pale orange | 0–5 ppm | Excellent |
| Light orange | 10–20 ppm | Good |
| Orange | 20–40 ppm | Monitor |
| Dark orange | 40–80 ppm | High — water change needed |
| Red-orange | 80ppm+ | Very high — act now |
Safe levels: Under 20 ppm for most freshwater fish · Under 10 ppm for shrimp, discus and sensitive species · Under 5 ppm for planted tanks
pH
pH measures how acidic or alkaline your water is on a scale of 0–14, with 7 being neutral.
| Colour | Reading | Suitable for |
|---|---|---|
| Yellow | 6.0 | Softwater species, blackwater biotopes |
| Yellow-orange | 6.4–6.6 | South American species, planted tanks |
| Orange | 6.8 | Mixed community tanks |
| Red-orange | 7.0 | Wide range of species |
| Red | 7.2–7.6 | Livebearers, goldfish |
For African cichlid tanks and marine systems, use the High Range pH test which covers 7.4 to 8.8.
What to Do When Results Are Bad
| Reading | Immediate action |
|---|---|
| Ammonia above 0 | 25–50% water change, find and remove source, do not feed |
| Ammonia above 1 ppm | 50–75% water change, use ammonia detoxifier, retest in 4 hours |
| Nitrite above 0 | 25–50% water change, check filter is working, add aeration |
| Nitrite above 1 ppm | Large water change immediately, aquarium salt reduces toxicity |
| Nitrate above 40 ppm | Water change to bring below 20 ppm |
| pH crash | Do not make sudden corrections — gradual adjustment only |
Why Your Readings Might Not Be Accurate
Even following the instructions correctly, manual colour comparison has real limitations. Lighting conditions, background colour and human colour perception all affect readings. The nitrate test is particularly prone to errors from insufficient shaking.
AquaBuddyAI uses CIEDE2000 colour science with a locked camera to eliminate these variables — giving you consistent, reliable readings every time regardless of lighting conditions.
Know what your readings mean for YOUR tank
AquaBuddyAI reads your test kit and tells you exactly what to do based on your specific setup. Free to download.
