Most Nitrite poisoning cases I see in beginner tanks are caught late because the early signs are easy to miss under normal lighting. The good news: most cases are treatable at home if you act in the first few days.
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 Aquarium LED Lighting Guide: Kelvin, PAR and Photoperiod makes the numbers obvious.
The treatment
Step by step:
- Move the sick fish to a hospital tank if you can, see Best Aquarium Heater 2026 (and How to Size One).
- 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 25% water change every other day.
- Follow the medication label exactly; overdosing does more harm than the disease.
Catching it early
Catching Nitrite poisoning early means watching for quiet clues, less interest in food, clamped fins, odd behaviour under normal light. Test the water the instant something looks off; early action beats a late scramble.
The symptom
Fish gasp and turn dark; gills may look brown. Nitrite blocks oxygen uptake.
Prevention
Quarantine every new fish for 4 weeks, keep nitrates under 20 ppm, and avoid overcrowding. Stability is the best medicine, a tank that holds its parameters rarely sees outbreaks.
