If you only learn one thing about Nitrite poisoning, make it this: the visible symptom is rarely the real problem. The good news: most cases are treatable at home if you act in the first few days.
The symptom
Fish gasp and turn dark; gills may look brown. Nitrite blocks oxygen uptake.
What is actually causing it
Ranked by likelihood:
- Stress from poor water — ammonia or nitrite above 0 ppm, or nitrates over 40 ppm.
- Secondary infection — a wound or parasite opens the door.
- Temperature swing — more than 2 °C in a day weakens immunity.
Test the water first; treat the cause, not just the spot. A a solid lymphocystis: the 'cauliflower' wart virus in fish makes the numbers obvious.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see our brine shrimp hatchery guide guide.
- Raise the temperature slowly to 28 °C to speed the pathogen's life cycle (species permitting).
- Add 0.2% aquarium salt, dissolved first, and keep it stable with a 27% water change every other day.
- Follow the medication label exactly; overdosing does more harm than the disease.
Prevention
Quarantine every new fish for 3 weeks, keep nitrates under 20 ppm, and avoid overcrowding. Stability is the best medicine, a tank that holds its parameters rarely sees outbreaks.
A step-by-step plan
Step one: test the water and fix any ammonia or nitrite. Step two: move the fish to a hospital tank if you can. Step three: follow the medication label exactly near 30 °C. Step four: re-test every other day.
