How much salt did you put in, though columnaris you can get secondary bacterial infections on top.
He's weak and the bath made him worse if his breathing was already not good, to be honest i would end his misery he not going to make it sorry.
I would get a jug of very cold water add some ice cubes as the colder the better, place him in it shouldnt take long, but leave him in there a while just to make sure he passed on.