New aquarists add fish before bio-filter establishes. Ammonia spikes kill within days. Cycle 4-6 weeks before stocking.
New fish bring ich, velvet, flukes. Disease wipes out the tank. ALWAYS QT 14-30 days.
Declining biofilter, accumulated nitrate, mineral drift in tanks 12+ months without water changes. Reset with weekly 25-30%.
Heater stuck on (cooked fish), heater failed off (cold), filter dies (ammonia spike). Test equipment monthly.
pH/GH mismatch for species. Always research species parameters BEFORE buying.
Bullying causes stress + secondary infections. Watch interactions; separate aggressors.
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 disease database if illness is suspected.
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