PetSmart and Petco are the devil. I wanted some flash and color for my 40 gallon, but nothing too overly demanding, so I snagged some glolite tetras. I asked the "Aquatic Specialist" for 8... and she started to net pristellas.
Another time, a different employee told someone that ramshorn snails won't reproduce in their aquarium.
Although, as far as fish conditions, my local Animart is worse. They get in some COOL. AS. HECK. fish sometimes. Like woah. But they're all COVERED in ich. No matter how awesome a fish is I won't buy anything from there anymore. Every time I do, it's weeks of medicating my tank to get rid of all the various diseases.