Kenji,

I think you should change 50 to 60% of water, put in anti-chlorine, and add salt to the water, recommended dosage is 0.3% of salt in the water(1gram of salt to 1 liter of water, make 0.1% of salt) so you should add until 0.3%

Try to put some airpump and airstone to help him breath and don't put too strong water pumps, it will make the fish dizzy, just gentle flow will do.
And please don't put any medication if no sign of sickness appear, sometimes wrong medication will kill the fish, so put only salt will do.

Hope you're Luohan will be OK!

Regards,
John