A vet would give you a dose you would have to add it to the tank, sometimes antibiotics don't tend to wipe the entire bacteria colony out in the filter, or he might tell you to mix it up in there food the antibiotic, but i would issolate the sick one as he dosn't look to good at all and to be honest i think i would destroy it, as you don't want the fish in the tank pecking at his dead body.