lol, why do they say it tho, when i started off with my baby biorb(15l) the bloke at forty fathoms in kidderminster told me that i could keep 3to4 small goldfish with 4 wcmm, i believed him, i was new to the hobby and thought he would know, do they say it so things go wrong so that u have to keep spending money at their shop??
I suspect that may be the case sometimes, but I think it's often a case of old and outdated information still being used . The old " a fish will only grow to the size of the tank " myth that kills so many goldfish every day, or " old water is best ". That sort of thing. If it isn't older shopkeepers giving out such outdated info and practices, then it tends to be younger people who are working in the stores to earn a bit of extra cash who either don't know a thing about fish, or are going by the previously mentioned outdated advice. ( this often occurs in chain stores )
Obviosuly not all of them are like that. But we all have at least 1-2 bad stores where we live who give out such advice to newbies.
At the end of the day if a couple come in with a kid who wants a goldfish ( and who will invariably get bored of it in a few weeks ), they are unlikely to buy one if you tell them how big goldfish grow, how long they live, what size tank they need and how much care needs to be taken to keep them healthy and large. Not to mention how you have to cycle the tank for a month before you even add a fish to it. Little Brandon/Lucy/whatever wants their fishy and they want it
now in a pretty little bowl or one like a spaceship. If it dies in a few weeks ( due to ammonia poisoning for example ) , oh well we'll get an identical one they only cost a couple of quid.
And it happens often enough for the shopkeper to make money from the fish. The people keep on losing fish due to lack of research and/or bad advice, and the guy running the shop gets to make money selling them cheap fish at a few quid each every time.
It's a business and it needs to make money. Unfortunately many fish have to suffer for the financial gain. Sad but true.