I have a full-time job during the week, but work weekends at a department-store sized pet store. Our fish room employees are, for the most part, very knowledgeable and our goal is to educate customers - not just to sell fish.
I generally ask customers how large their tank is, what they've got in the
tank ATM, and take it from there - if they want something like an oscar, pleco, bala shark, gar, etc. I'll ensure that they know the potential size of the fish and have a large enough tank. If they're having problems we break things down; I'll ask about their maintenance routine, tank size and inhabitants, how long the tank has been running, and anything else I need to know in order to help them pinpoint where their problem (algae, disease, mysterious deaths, incompatibility issues, etc.) stems from.
The worst customers, IMHO, are those who are determined that they're right, period, no matter what the issue. There are lots of different kinds, but they all share the common quality of being experts in their own minds. Some insist that keeping six oscars in a forty gallon tank is fine and not the cause of their ammonia and agression problems, others think using trios of goldfish as a table ornament at a wedding is fabulous - even after I explain that keeping them in those adorable, teacup-sized containers is likely to result in tables full of dead goldfish stinking up their special day. Some like to give other customers advice that conflicts what we're offering, even butting in to interject their opinions. After a while, I learned to allow these people to believe what they like and just go on with my day - if I didn't, I'd stay mad all day.