Simple definition: Nitrite is the middle stage of the nitrogen cycle, produced when bacteria break down ammonia, and it’s still toxic to fish.
Detailed Explanation
A second group of bacteria converts nitrite into nitrate, which is far less harmful. Until that second group is established, a pond can show a ‘nitrite spike’ even after ammonia has dropped, which is a common trap in new ponds.
Why It Matters
Nitrite interferes with how fish blood carries oxygen, so fish can show oxygen-starvation symptoms even when dissolved oxygen levels look fine.
Practical Example
Testing only for ammonia and assuming a pond is safe once it reads zero is a common mistake. Nitrite often rises right after ammonia falls.
