6.2 for white clouds? Well, if the store has a more normal ph (7.0) or slightly alkaline, then the introduction of these white clouds should have taken a much longer time. Hours would have been good via a drip method. White Clouds are very tough - but a sudden jump from their usual PH to your acidic PH would have been a shocker.
Next time try this:
When you bring them home, put them and the water they arrive in into a container. Every 10 to 15 minutes, add a small amount (tablespoon or so) of your tank water to their tank. Eventually, you want to double or triple the amount of water in their container. Then, net them out of the container, and place them in your tank. (Hopefully water temps are within a degree or two of eachother).
If they die after a slow transition like that, then I'd be surprised. But 6.2 is too low for what white clouds actually like long term. And a shocking ph change might have done it.