Most common cause. Test temperature - if below species range, the heater failed. Fix: replace heater, gradually warm water 1-2°F per hour.
Acute toxicity causes fish to lay on bottom. Test immediately. Fix: 50% water change + Prime.
Late-stage ich, velvet, dropsy, or columnaris. Fish weak from illness. Treat in QT or euthanize humanely.
Fish nearing natural end of life lay still + eat less. No fix - keep parameters stable + water clean.
Plecos, loaches, corydoras lie on bottom while sleeping. Normal if they swim normally + eat normally when active.
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.
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