I have a paroon shark (he was still in his quarantine tank) and yes I do know how large they grow !! 
Anyway he has been fine, eating happily etc etc
I did a 25% water change Thursday night
Friday night he was perfectly normal, but saturday morning he was struggling to swim, floating to the surface, swimming upside down(when he could swim) small mouth openings at the surface
The filters sort of pushed him around the tank a bit, no visbile signs of any thing wrong with him at all
He was and has been the only fish in the quarantine tank, after about half an hour of this he gave up and died
I did the water checks, Ammonia, Nitrite & Nitrate levels were all perfectly normal
Ive attached a pic of him, just after he died, but I cant work out why, as it appeared so suddenly
Steve
Anyway he has been fine, eating happily etc etc
I did a 25% water change Thursday night
Friday night he was perfectly normal, but saturday morning he was struggling to swim, floating to the surface, swimming upside down(when he could swim) small mouth openings at the surface
The filters sort of pushed him around the tank a bit, no visbile signs of any thing wrong with him at all
He was and has been the only fish in the quarantine tank, after about half an hour of this he gave up and died
I did the water checks, Ammonia, Nitrite & Nitrate levels were all perfectly normal
Ive attached a pic of him, just after he died, but I cant work out why, as it appeared so suddenly

Steve