I think Never is a over statement, some are really good and helpful you just need to find the right place. in regards to
LFS tanks being overstocked they can get away with it more then Us (Home users) if you notice sometimes you see that all thier tanks are connected in some way which in turn is connected to a super filter (just a really good one). although like you said i havent seen this place and they may infact be overstocking the tank.
heres just a little story of my fish experience, when i went to choose my first fish i choose this guppy although the store owner rejected me and said that one has been bitten choose another. he was a top person. another shop sold me tiger barbs and told me they would be friendly with my other fish wrong wrong wrong. he lost me as a customer now.
the moral of the story is not all LFS employees are bad they are some good guys out there
