Most common - livebearers (mollies, platies) need GH 10+ but kept in soft water. Brain shock causes shimmies.
Some fish (cichlid males, bettas) shimmy to attract mates. Normal if accompanied by color flashing.
Velvet causes neurological shimmies. URGENT - copper treatment immediately.
Pesticide, soap, or chemical contamination triggers shaking. Massive water change + carbon.
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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