Most common cause. Caused by overcrowding, dirty filter, low surface agitation, or warm water (warm water holds less oxygen). Fix: increase surface agitation with a powerhead or air stone, do a 25% water change.
Detectable ammonia (any reading >0) damages gills + makes fish gasp. Test ammonia immediately. Fix: 50% water change with parameter-matched water + dose Seachem Prime.
Bettas, gouramis, hatchetfish, killifish naturally hang at the surface. Normal if they swim normally + eat normally + show no other symptoms.
Parasitic gill damage causes oxygen-stress behavior. Look for white spots (ich), rapid breathing, scratching. Treat in QT.
Above 84°F drops oxygen capacity. Cool tank (see /how-to/lower-aquarium-temperature/).
Test water first - many "behavior" issues are actually water-quality problems. Use the water parameter checker to score your test results, the disease symptom matcher if you observe physical signs, or the general diagnoser to narrow further. Browse the full disease database if illness is suspected.
Browse all behavior troubleshooting articles.