Ammonia poisoning happens when NH3/NH4+ levels exceed 0.25 ppm and damage fish gill tissue + nervous system. The single fastest way to lose a tank.
Filter failure, tank crash, dead specimen decomposing, overfeeding, replacing too much filter media at once, or a new tank that hasn't cycled.
Always treat in a separate quarantine tank.
Test ammonia weekly. Don't replace filter media all at once. Don't overstock or overfeed.
Gasping at the surface. Red or inflamed gills. Lethargy and laying on the bottom.
Filter failure, tank crash, dead specimen decomposing, overfeeding, replacing too much filter media at once, or a new tank that hasn't cycled.
Immediate water change: 50% water change with dechlorinated, parameter-matched water. Continue 25% daily until ammonia is 0.
Test ammonia weekly. Don't replace filter media all at once. Don't overstock or overfeed.
CRITICAL - hours from problem to mass die-off
Browse the full disease database for 45 aquarium conditions with treatment protocols, or check the care library for prevention-focused husbandry guides. Use our symptom matcher to rank likely diseases from observed signs, the water parameter checker to diagnose related water-quality issues, or the QT timeline calculator to plan a treatment schedule.