Nitrite and nitrate differ by one letter and by roughly everything else. Confusing them leads people either to panic over a harmless reading or to ignore a dangerous one.
Where each one comes from
Your fish excretes ammonia. One group of bacteria converts that ammonia into nitrite. A second group converts the nitrite into nitrate. Nitrate accumulates until you remove it with a water change.
That is the whole cycle, and the position in the chain explains the difference in danger.
| Nitrite | Nitrate | |
|---|---|---|
| Stage | Middle | End |
| Target | Zero | Under about 20 ppm |
| Harm | High. Blocks oxygen transport | Low. A slow stressor at high levels |
| Removed by | The bacterial colony | Water changes, and partly by plants |
| A reading means | The cycle is broken or incomplete | Normal, unless it keeps climbing |
Nitrite, the one people forget
Everybody knows about ammonia. Nitrite gets less attention and is nearly as dangerous.
It interferes with the blood’s ability to carry oxygen. The practical consequence is strange and worth recognising: a fish in nitrite can gasp at the surface in water that is perfectly well aerated, because the oxygen is there and the fish cannot move it around.
A nitrite reading above zero means the same three things every time. The tank was never fully cycled, or the colony was damaged, or something is overloading it.
The response is identical to ammonia: 50 percent water change now, dechlorinated and temperature matched, feed less, repeat daily until zero, and leave the filter alone.
Nitrate, the one you manage
Nitrate is the end of the line. Nothing in a normal aquarium consumes it in quantity, so it builds steadily between water changes.
This is most of the reason water changes exist. Removing 25 percent of the water removes roughly 25 percent of the nitrate.
Under about 20 ppm is comfortable. Climbing past that is not an emergency, it is information: your changes are too small, too infrequent, or you are feeding more than the tank can process.
Live plants use some of it. They are a help, not a substitute, and a planted tank still needs the same weekly routine.
The reading that fools people
Zero ammonia, zero nitrite, zero nitrate, on a tank set up three weeks ago.
That is not a pristine tank. It is usually a tank where nothing has started. A working cycle produces nitrate as its end product, so the presence of nitrate is evidence the colony exists.
On an established tank, zeros on the first two with some nitrate on the third is exactly right.
Testing
Liquid kit, daylight, white background. Weekly once established.
For nitrate specifically, shake the second reagent bottle hard for the full time stated, then shake the capped vial hard for the full time stated. Under-shaking is the most common cause of a nitrate reading that looks impossibly low, and it is why some people believe a tank with no water changes has no nitrate.
Where to go next
The water parameters guide covers every reading in one place. The ammonia article covers the first stage, cycling covers building the colony that handles both, and the water changes guide covers removing the nitrate.