^^ I agree, they will be pissed but most petco/petsmart/major chain employees don't know and don't care what they know, this activity is just a waste of their time to procure enjoyment to those that find it funny.
I used to waste time watching the fish in such stores waiting for customers to ask for advice, THAT is where I got my enjoyment/torture because I would offer some real advice. Most of the people in those stores want an "easy" pet out of fish, something cool, colorful, large, active and extremely low maintenence.
I work in an LFS so I feel dirty trolling like that now and sick seeing some of their starving/dying fish.
When I did troll though, UGH, I heard all the same things I hear now except now I hear them from another LFS' standpoint. Just 2 days ago I had a couple that couldn't understand why the 20 fish they bought for a 10g a week ago all died.. turns out they didn't dechlorinate the water, didn't cycle the tank, didn't have a heater, mixed tropical fish with several goldfish and couldn't understand why it went wrong. The store had offered absolutely no advice, warnings, tips or anything.
AFTER I explained things, why they were left with only a pleco surviving.. I suggested they exchange the pleco, here or there, and recommended a few fish that maybe they could take care of on the *one* flake food they had. Unfortunately, fish are marketed as easy pets to have and people that have started won't hear otherwise. I tried to explain the nitrogen cycle and they pretended they heard more than 2 words of it. Fact is, MOST people won't be bothered to do things right and all you can be left with is the hope that they will come back to someone who will tell them what they did wrong.
Those people probably will kill all the fish I sold them. I know that and I feel terrible for it. However, if I refuse to sell fish to a customer they almost always go to the manager (and he happens to be the owner) and he will chew my ass out for refusing a sale. If they don't go to him, I know they're going back to one of the stores that set them up to fail and kill fish in the first place!
I DO warn people that fish will grow to X feet long and need a 300 gallon tank within a few years, I DO tell them it will hurt/kill their other fish, I DO tell them it is a BAD choice for their setup/size/community/other aggressive fish/etc. but people don't care.
That's really it, people don't care. They want it and they want it NOW and no minimum wage petsmart job is going to do anything but set the fishkeeping world back, especially with people in "testing" them and pointing out how useless they are.
I've been tested. Sadly, I did pretty good. I've found people who "test" LFS' on a regular basis WANT them to fail the longer they've been doing it. They have a bitterness and will seek out any shortcoming the person has even if they know their stuff.
I had a person in a few weeks ago "testing" me that first wanted to buy a fish so I bagged it. Then they started asking about other fish, and more fish and more fish, ALWAYS fish that were not compatible with the one I had bagged or were far too big for the tanks he described having.
Eventually we got to the point where his own stories were running together and I got pissed off and called him out asking "Exactly how many cycled tanks of what sizes do you have and what lives in them? Answer that and I will tell you what fish you could potentially add!"
Well, after wasting an hour and a half discussing all manner of fishkeeping with a seemingly interested individual it turns out he feels he knows far more than I could ever teach him and he was "just testing to see if I knew my stuff".
Do you have any idea how insanely irritating that is?
It makes it harder and harder to give that attention to people that really DO want to learn and have the potential to be diehard fish enthusiasts if someone would just point them in the right direction.
"Yeah, I just wasted almost 2 hours on some guy who claims he knows more than I ever will, here's some ammo-lock and a couple oscars for your 10 gallon.."