I work at the dreaded Petco as the aquatics specialist. =) I'm 22 and fairly new to the aquatics world, but I have a very good mentor that's been doing it for 9 years that I learned a lot from. I also must say that I come on here and read almost new threads every night for a couple hours on the days that I have time (probably 4 days a week). I guess most importantly I realize (and admit to) several of our customers are much more knowledgeable than myself and I enjoy learning what I can from them.
When I talk to customers, I always ask them how big their tank is, how long it's been set up and what type of fish they have. If by this time I can tell that they really have no idea (if the conversation goes "What size tank do you have?" "Uh.. I dunno, the regular size." and they describe their fish as the "red kind", I know they have no idea) I also ask them what their temperature usually is, how often they do water changes, etc and help educate. I've had so many people say "Well, we changed the water a few months ago, took all the gravel out and cleaned it all really well." I definitely try to kindly educate these sort of people.
The people that come in and know exactly what they want and can actually competently answer a few questions, I pretty much give them whatever they want and have great conversations.
In the case of people that demand four goldfish for a ten gallon tank (happened to me just a few weeks ago, actually), I try to kindly explain the nitrogen cycle, the fact that their fish will get around a foot long, and it's definitely NOT recommended. If they still demand them, I bag them up. Some times I tell them the guarantee is void, but in other cases I don't. They have to have a water sample anyway and if the water isn't good, the guarantee is void as well. So it really just depends on my mood, I suppose.
Most of the customers really appreciate the advice and education and are happy! Those are the best. They really are one of the biggest reasons I am still at my job. I absolutely LOVE teaching people about water changes, cycling, and fish. Nothing better than taking someone so tired of their "dirty algae ridden tank" that they're ready to trash it and help them get excited about turning it around for the better just by providing some information.
I also think there is a difference in our average customer vs the average customer at an aquatics specialty store. A lot of ours are the type of people that just come in, see the fish, their kids go nuts over them or they decide to try them out. We have a handful of serious hobbyists since we're the only fish store, but the majority don't do much research at all.