petstores are based on making money, iv'e worked for many and i decided that it wasnt my best intrest in working there for many reasons, the above post are great examples of things to learn and do, but all in all it's about the sell. i learned along time ago that people want fish, they want fish to live but dont want to learn how to keep them alive.. there is no easy answer in these cases, just suggest as best as you can all the proper ways / sizes of fish in tanks, what goes together and what doesn't ..ect
but and you will find this everyday you work people will want unreasonable things like a bala in a 10gl tank, "my suggestion is just swallow you pride and after you warn them" SELL THE FISH... i know how this sounds to so many but people here care,.alot of people that shop for fish dont care at all, it's a supply and demand world,. + if lets say the bala does live in a 10gl tank for them, then you have a better chance of selling them a larger tank, or having the fish returned to the store to be sold again for more money for the store,making you good at your job from the point of you boss,.
remember fish stores are about selling fish, not refusing to sell fish because you know they will die ,. give the customers a chance to decide, if they make the wrong choice, dont let it get you down,. im not saying lie to anyone just sell them what they want, you boss will apprecate you more if your making him money. dont lie about fish but also if they dont want to learn dont feel bad about making the sell anyways.. sorta like if they didnt ask / dont tell .. just an opinion,. good luck.
knowy