This might sound off the wall, but if it is not a pH swing, you might have some sort of parasitic or fungal disease (the white patches you describe?) that only affects the Harlequins. The South American fish (even the relatively delicate Cardinals and Neons) might not be affected.
Maybe try treating with some broadband antibiotics just before you add the new fish and see if they survive the rigors of acclimation this time.
- Sam P -
plantedtanker in limbo - all tanks currently in storage