Nitrite is the middle step of the nitrogen cycle. Bacteria in the filter turn toxic ammonia into nitrite, and a second group of bacteria then turns nitrite into the far less harmful nitrate. In a mature, well-run aquarium nitrite should therefore never be measurable. When it does appear, something in the biological balance has gone wrong.
Why nitrite harms fish
Nitrite enters the fish through the gills and reacts with the red blood pigment that carries oxygen. The altered pigment can no longer transport oxygen, so a fish can struggle for breath even though the water itself is well oxygenated. In serious cases the blood takes on a brownish tint, which is why nitrite poisoning is often described as “brown blood disease”.
Typical signs
- Fish gasping at the surface or hanging near the filter outlet
- Fast gill movement and lethargy
- Loss of appetite
- Pale or brownish gills
Common causes
- A new aquarium that has not finished cycling
- Too many fish added at once
- Overfeeding and decaying food
- Filter media cleaned too hard or replaced all at once
- Medication that has damaged the filter bacteria
- A filter that has stopped for several hours
What to do
- Test the water and carry out a partial water change straight away, using conditioned water of the same temperature.
- Reduce feeding for a few days so less waste reaches the filter.
- Do not clean or replace filter media while nitrite is present.
- Support the filter with live nitrifying bacteria such as LGND Medium, which helps re-establish the biological filter quickly.
- In an emergency, a product such as LGND AmmoProtect temporarily detoxifies ammonia while the bacteria catch up.
Where the fish species tolerate it, a small amount of aquarium salt can also help: chloride competes with nitrite at the gills and reduces how much the fish absorbs. Check first, because many soft-water fish, plants and invertebrates are sensitive to salt.
Prevention
Cycle a new tank fully before adding livestock, stock gradually, feed only what is eaten in a couple of minutes and rinse filter media gently in old tank water. Regular testing is the best early warning: nitrite should always read zero. For new tanks, LGND Entity helps break down organic waste before it can overload the filter.