I believe gas bubble disease is similar to the bends. The more I think about it, the less I think it's the problem.
Unfortunately, I didn't check the fish store pH, but I'd be surprised if the pH was that different. The fish store uses the same water that I do for their tanks, but I will ask...
I just purchased new fish - 2 platy, 2 koi swordtails, 2 small frogs and 1 glass cat. As soon as I added the fish, all four livebearer showed signs of distress: sinking, rapid gill movement, fins slammed back. The two swordtails were the worst. The male fell over and had bright red gills...