While it's true that you're generally better off buying fish from a reputable source, it's not a guarantee. An awful lot has to do with whether and how you quarantine. No matter where your fish come from, you should always quarantine them. The parasites they often carry set them up for secondary bacterial and/or fungal infections. If you treat just for those infections and don't get rid of the underlying cause, the fish will die.
The moor you see? Three dollars at Meijer, which over here is one of the worst places you can buy a fish. Their tanks are terrible. But every once in a blue moon, you'll see a fish with some potential. And because I like a challenge, I'll buy it as a project to show what's possible. They go straight into quarantine and get treated for the parasites they almost certainly have. Lots of water changes, good filtration and proper feeding. They stay there for a minimum of a month, usually more. Only when I'm certain they're healthy will I move them in with other fish. That particular one weighed ten grams when I bought it in January, it's now just shy of fifty. What I do is no guarantee, either, but my goal is to stack the odds way in favor of the fish's health.
