Yea, I had a bunch of cardinals that had no probs at all. Until I moved them into a new tank. It was fully cycled, checked, the usual, but killed every fish except 1 that I put into it, coz I got it moved sharpish...
Got money back for the tank, but lost so many fish. No-one had an explanation.

But not to worry, got a new, bigger tank and so far doing well. My new neons are loving the tank they are in at the moment, but are being moved to the big tank soon once fully cycled and I'm sure they will love it there too (they will have bubbles in there too, oooohh!!!)