Hi SW - i hope you haven't put him in with your other fish just yet. To protect all fish involved, it's best to quarantine each new addition (as shj mentions) and salt to 0.3% (1 teaspoon per gallon (= 0.1%) x 3 times - 12 hrs apart) but there's a bit of work involved which is why IMHO it's handy to have a quarantine tank ready cycled. This way, if any new fish is harbouring anything, they get the benefit of perfect water conditions to build up their immunity against bacteria (and other nasties) - plus it's much easier to treat
In very rare cases, fish may come from an outlet healthy - but the thought of taking that chance makes me shudder. One fish is much easier to cure then a few and you don't run the risk of losing any of your healthy little fellas