Racoon butterfly problem, Need help !!!

[barracuda]

Hi all,
I have only 2 fish in my FO tank. Cream Angel and Racoon Butterfly. For already 2 days this fish is swiming on 1 side. The fish is eating well, but this is not his regular behavior. Some info: a week ago my Lion fish is died because of unknown reason. Ammonia, Nitrite, Nitrate an PH are OK. Tank is 37 gal, Cannister biofilter and MERLIN fluidization filter.

 

[barracuda]

I have the Racoon for about 1 month. The tank is running for 3.5 month. Lion fish was acting normaly before he died, i mean eating very well and swiming. The tank is full of live rocks, so there is a lot of space to hide. The racoon seems to be just loosing the balance by swiming sideways instead of upright. The breathe is normal.