In my short experience I've been to 3 LFS 3 Petsmart and 2 Petco stores. Of which only 1 Petsmart was good enough for me yo even stand there and watch the fish. And that's where I got my Orandas. I don't even want to talk about Petco. And the LFS near my house for some reason always has sick Orandas. Rest of the fish seems to be okay, but the store stinks. The girl working there told me just buy the fish put a filter and you should be good !!! So I don't think LFS by definition means they know what they are talking. IMO if a hobbyist is in this business then he/she might do better a job of caring for the livestock. If it is a pure business person then they might treat the livestock as just an inventory than a living thing.
Back to OP's mission, being an hobbyist you will be a better person to deal with the livestock. At the same time just look at the overall culture of that business and tune your strategy to achieve your goal. As others said, you don't want go to extreme right away and loose the opportunity. Good luck in your mission.
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With you being a hobbyist you may grow frustrated with the way people treat their fish. Just try to compartmentalize that and try not to let it bother you. A brief education may work but some people will just do what they want. Customer service is key, get the employees trained up on knowledge.
Also as a previous poster said if you deny a sale to someone, you basically just lost a customer and they will probably not come back because they may feel embarrassed. However if someone comes back every 7 days with a dead fish, maybe kindly ask for a water sample before you sell them another fish.