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.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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