fish shaking his head

Question
my sevrium cichlid has begun to shake his head with quick jerky movements. Transferred him to established 55 gal. few weeks ago. Had a large peacock cichlid die several months ago in 55 gal. I removed all water, bought new canister filter,cleaned tank well including water polishing,kept old biofilter going, and let water age for 5 weeks. No other symptoms. Any idea whats wrong? He (or she, how can you tell?) is at least 5 inches long and a beautiful lemon yellow. I would  to lose him. He lives with a red tailed shark (4") and keeps the shark in line. Thanks for your time!!!

Answer
Hi Kev,
 I don't think anything is wrong.  Quick jerky side to side motions by something like a severum is a display, typically indicating to another fish that the severum means business.  

-- Ron
  rcoleman@cichlidresearch.com
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