when your at the shop buying them look very closely to make sure thewre's no sign of whitespot, if there's 1 spot don't buy, but don't get paranoid about it!!
get them from a a shop that has good catchers aswell, may sound stupid but it does no good the fish being chased aorund with a net for 5mins by a stupid imployee.
take your time to acclimatise them, atleast 25 mins, turn your lighting down to a minimal ammount.
then hey presto you should have whitespot free clown loach!!
although if you don't, i'ved treated clown loach succesfully with Interpet No. 9f at half dose.