API Nitrite Test KitThe API Nitrite Test Kit measures nitrite (NO₂), the second toxic step in the nitrogen cycle. After beneficial bacteria convert ammonia into nitrite, that nitrite is still dangerous — it interferes with a fish's ability to carry oxygen in its blood, and high levels are rapidly lethal. Watching nitrite climb and then fall to zero is how you know the second half of your cycle has matured.This liquid test is precise and economical, good for nearly two hundred tests. It's indispensable during cycling and a smart periodic check afterward, since a nitrite spike is one of the first warnings that a biofilter has been disrupted.Good to Know: Nitrite is a separate parameter from ammonia — a zero ammonia reading tells you nothing about nitrite, so both must be tested while a tank establishes. On a healthy, cycled tank, nitrite should read 0 ppm at all times; any reading above zero calls for a water change and a look at what changed.How to Use: Fill the tube to the line, add the specified drops of nitrite reagent, cap and shake, wait the stated time, and compare to the chart. Test every day or two during cycling and after any major change (new filter media, a deep clean, a power outage); otherwise a periodic check is plenty.
