The classic beginner story goes like this: buy a tank, set it up, fill it with water, and — the same day, or the next — add a hopeful little crowd of fish. Within a week or two, the fish start dying, and it reads as bad luck or bad fish. It is neither. It is the predictable result of skipping the one idea that the whole hobby rests on, the idea no one mentions at the checkout: you are not keeping fish. You are keeping water.
The thing nobody explains
A new tank of water is not yet a place a fish can live. Fish produce waste, that waste turns into substances that are toxic to them, and what makes those substances harmless is a population of beneficial bacteria that has to grow and establish first — a process that takes weeks, not minutes. Adding fish to brand-new water means dropping them into a system that has no way to process their own waste, and the waste poisons them. The water has to be made livable before anything lives in it.
Why the shop won't tell you
Because "go home, set up your tank, and wait several weeks before buying fish" does not sell fish today, and because it requires explaining an invisible biological process to someone who just wants a pet. The result is a hobby with a brutal beginner failure rate driven almost entirely by a single missing concept that nobody had the time or incentive to teach.
The honest start
Set the tank up, get a way to test your water, and establish the system before you stock it — let the biology catch up, add livestock slowly, and treat stable water as the actual goal. Learn to keep water and the fish largely take care of themselves. Skip it and you are not unlucky; you are drowning fish in poison you could not see. Keep the water, and you keep the fish.