you have to treat the whole tank, yes, because most of the parasites will actually live in the water, and it is when they are in the water, not on the fish that the salt or medication will be effective. this is one of the few diseases where you shouldnt quarantine for that exact reason.
i have just had itch and had a Pictus Catfish
in the tank which unfortunately the salt, even at a lower dose because of him, he died
i dont know what medicines would be available in your area but i used protozin
and it was very effective. it is also safe for scaless fish. i cant tell you what is in it so definitely
do research before buying it as like i said i dont know what the ingredients are therefore dont knwo how suitable it is for your fish. im just letting you know that its out there really.
another medicine i was recommended was formalin
. you could check this out too. it was bettababy that recommended this medicine to me and she was very helpful, sending me pms about the medicine. im sure she would not mind a pm from you as well if you want information about it.
another thing is to raise the temperature a few degrees while treating for itch as it speeds up the parasites life-cycle therefre getting them to the stage where theyre in the water quicker which means the parasites can be killed off quicker.