Now I don't want to make it sound like all pet stores give bad information about fishkeeping, because some are very good at what they do. However, it's been my experience that most pet stores give pretty bad advice when it comes to proper aquarium fish care, so I'm going to be doing a mini-series talking about some things pet stores don't tell you about fishkeeping.
Video Link: https://www.youtube.com/watch?v=6W92KuT-T8A
I would say the most common thing pet stores don't tell you is the fact that you need to properly cycle your tank. Most of the time pet store employees will just tell you to set up your tank, add water and dechlorinator, and let the filter run for 1 week before adding any fish. They tell customers that this will "cycle" the tank. In reality, this does absolutely nothing. Cycling the tank means you're establishing beneficial bacteria which convert toxic fish waste products (like ammonia) into less toxic forms. Without a source of ammonia, the tank cannot begin to cycle. So if you're just adding water and running the filter, there is no cycling going on whatsoever. Furthermore, cycling the tank takes much longer than just 1 week. It actually takes around 3-4 weeks!
For more information about cycling your tank properly, please see this link: Fishless Cycle