i would says the most important things ive found to be are learning about the Cycling of a fish tank, and mostly about what fish go in what size tank. just because they fit in the tank doesnt mean they will be happy
do NOT attempt to explain how cycling works. I've tried a couple times, and people buying fish could not care less about this "complicated" subject. they'll just zone out any time you mention cycling after that. just touch on the important parts and say "toxic" and "new tank syndrome"alot lol. they're good headliner words. and tell them to take it slowly and bring their water in to be tested in a week.
suggest cheaper alternatives to your hardware. You'll gain loyal customers by directing people to amazon.com and drsfostersmith.com
if the customer is about to make a poor choice about stock because of your limited options, suggest fish that your store doesn't carry. i need to get some business cards for bobstropicalplants.com cuz i always suggest his nerites and have to write it down. I had a couple people come back saying how great their nerites or otos are, wherever they ended up getting them. it's nice to gain their trust, and it's good for long-term business.
don't be afraid to turn people away. sometimes it's easy, like someone buying a koi for his 10g, but sometimes it's hard, like the adorable little girl who wants to buy a goldfish who she has already named, but her parents will only let her have a bowl. Then she's all sad, and the mom is angry, then the little girl comes in later with her dad who asks again and is all like "why not?" and you're all like "cuz we try and make sure our animals get adopted into good homes." and he's like "but it's just a goldfish!" and then you pull a sly one and tell the little girl that Nemo won't be happy in a bowl and he'll get sick, and she should get a puppy instead of a fish. Then she'll want a puppy, and the dad is even more unhappy, but you'll be smiling on the inside.
Be prepared to explain to every little kid why there's a dead fish in the tank. "he lived a long and happy life, and he was old" is a winner.
always remember to mention water conditioner! also, ask why people are buying meds for the tank, sometimes they just pick up a random medicine hoping it'll work.
NEVER suggest petco brand filters. they'll be discontinued someday soon, and people will hate you for it because they can't find any cartridges.
maybe get on the computer in the breakroom and file a suggestion that they stop carrying common plecos and chinese algae eaters.