Ok I can try that out tomorrow. I’ll put the clown in an acclimation box and release the new fish so that he can get adjusted and if that doesn’t work then I’ll treat the tank to see.Yes, if it works then you probably isolated your source of difficulty.
Some folks get an acclimation box, I would put the clown in that and the new fish in the tank.
Some people put naughty fish in their sump. If it's safe to do so, while the new fish lives in the tank with the aggressor in the sump you could eliminate aggression as a variable. Again, this is only a guess. I have not seen your fish, the tank, etc so I clearly don't have all the info. Others are leaning towards disease, I think its stress/aggression possibly.