Fish gathering at the surface is a distress signal - identify the cause within 1-2 hours. Most likely: low dissolved oxygen. Causes: high temperature (above 82°F = lower O2 saturation), ammonia spike (gills inflamed = can't absorb O2), dirty filter (poor circulation), dead fish/snail decomposing, CO2 overdose in planted tanks, or recent medication that depressed O2. Immediate action: add an air stone, lower lid for surface agitation, do a 50% water change with matching-temperature dechlorinated water. Test ammonia + nitrite + nitrite + temperature + pH. Other causes: bettas naturally surface-feed (they breathe atmospheric air with their labyrinth organ). Some species (gourami, hatchetfish, killifish) prefer the upper third. Velvet or ich: if fish are scratching against rock + breathing rapidly + at surface, it's likely parasitic - move to QT and treat. Diagnose with our aquarium diagnoser tool for symptom-matched treatment guidance.
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