By all good, you mean zero ammonia, zero nitrite, and nitrates below twenty, right?
You ph isn't bouncing up and down? cuz that would be bad.
Look closely for signs of external parasites, especially on the gills.
Actually, if you went back to the lfs and other fish there were doing it, your fish may have been sick but not yet showing symptoms. I'd try to get my money back if there are fish with the same symptoms there. And, I know that doesn't help your other fish, but it sounds like you are at the point of diagnosis now, as long as your water chemistry is good.
Since you already have sick fish, are you sure now is the time to start monkeying around with the ph? Unless you are sure you can stabilize it, the up and down will stress them bigtime. Just something to mull over, that's all.