would have to agree with globaldesigns to put all your fish in quarantine now without any visible signs of any thing bad would only add stress if you wanted to play it safe just observe him if it is ich watch for white spots theyll show.
if you have to put your fish in quarantine you can drop the salinity quite fast its raising the salinity that has to be done gradually.
i do like hyposalinity but you really need to do it carefully in order to not make your fish stressed out.....stressed out fish hide and dont eat therefore cant fight off the attack.its never wise to treat your fish or tank in any way without knowing what the problem is.
pics of the fish close up helps also
cheers