Normal in first 3-7 days. Fish adjusts to new environment + tankmates. Should emerge gradually.
Bullying or territorial pressure causes hiding. Watch feeding times. Solution: rearrange decor, separate aggressor.
Some species (loaches, pleco, killifish) prefer dim light. Reduce lighting intensity OR add floating plants for shade.
Fish stress without cover. Add caves, driftwood, plants. Each fish needs its own potential hiding spot.
Sick fish hide. Watch for visible symptoms in next 3-5 days.
Ammonia or pH crash triggers hiding. Test water.
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.