At my local PetSmart, only the manager knows anything about fish. Everyone else is clueless, even after working there for several months. However, the tanks are well maintained. If they carried anything I was interested in, I would buy from them.
My local Petco constantly have tanks of dead and dying fish. Don't get me started on WalMart, and I like them for everything except live fish. I sincerely hope WalMart stops selling fish, as I consider them incapable of selling fish in a responsible manner.
I prefer non-chain LFS with an owner that spends his time in the store.
As always, your milage may vary.