"Everyday, in every way, I grow more and more like a fish.

At my job one of the greatest services we provide to the customer is we help them not to buy fish. For every 2 fish I sell I make sure another 5 remain safely in their tanks. Not that I particularly don’t want to sell the fish, its just sometimes you have to make a judgment call.

There are three main strategies of avoiding fish sales.
Distraction; for example, directing a customers attention away from the blood parrot he wants to put in his 20 gallon tank that is currently housing a thriving community of tetras.
Detail; for example, using a thorough and overly scientific explanation of salinity and osmotic potentials to that one costumer who just refuses to accept that he can't put a dozen guppies in the same tank as a chocolate chip starfish.
Hiding; Finding something to do at the other end of the store to avoid that one costumer who is completely convinced that they want one of those Nemo fish and no they won’t be getting a heater or sea salt thank you very much for suggesting it. (No matter how many times you say clownfish they give you a look and refuse to call it anything other than nemofish. Yes madam the scientific name is Nematias nemo; we all just call it a clownfish because we watch too much discovery channel on our off time.) With a person like that so long as you keep out of their sight they can’t buy the fish and, given enough time, they will lose interest and begin contemplating purchasing another poor doomed animal. It breaks down to a waiting game - me vs. them. My ability to stay occupied elsewhere vs. their attention span concerning the fish.

It is store policy to make sure the purchaser is capable of keeping the pet in good health, so these tactics are for the good of the customer. Sometimes you have to save them from themselves. That guy who wants to buy a dozen neon tetras and 5 tiger barbs for his already full mid-sized aquarium doesn’t want to waste money on fish doomed by his error and it is your job as an aquatics department servant to politely suggest a different purchase... that of ice cream from the Baskin Robbins next door.
You see, when the person doesn’t believe you that saltwater fish need to be in saltwater (and yes this is not uncommon) and you tried all three tactics, you might need the good old blunt force trauma to help them realize that they are on their way to becoming a serial fish killer. By blunt force trauma I mean something like this. “Well Sir/Madam the best advice I can give you about your tank right now is to just go ahead and turn around, walk out the front door, grab an aquarium manual on your way out and stand in the parking lot. Breath deep and say to yourself. ‘I’m not a bad person; I’m just opinionated and ignorant.”

And, if you were curious as to common errors made by nubies, here you go:
Overcrowding; it’s a five gallon tank and they have a shoal of neon tetras (these fish have to work in tandem just to make room for one of their brethren to turn around).

Improper equipment; It’s the Pacific Northwest and they have a bunch of fish from the South Pacific and no heater. (This is an easy one to point out. “Ever been to Maui? You have, good. Remember how nice it was and how warm the water was? Now think about your house and how you don’t need a refrigerator to keep your drinks cold. I rest my case. Buy the damn heater).
Lack of Foresight; Buying the cute little fish for a tank without bothering to find out how big it will get. And then having to face the difficulty of a fish that only is able to keep a fourth of its body in the tank and submerged at any given time. (If the customer says the plan to release over sized fish into a nearby lake, I like to ask if there are any other pets we can provide to them for illegal uses - Ferrets for minesweeping perhaps.)
Lack of commitment to the tank. If the customer says they had a Betta but gave it away because it was too much work and they are looking at saltwater aquariums, I just sigh, shake my head, and point to the front door and thank them for stopping by.

I suppose it’s the same with any animal. Reptile stores must have the same complaints.
“Oh I want that little snake for my son.”
“Lady it’s a Reticulated Python.”

She chose poorly.