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=NtLpJvVKFyI
In my experience, many pet stores don't properly educate people about the importance of regular water changes. For a normally stocked goldfish tank, you'll want to change at least 50% of the water once a week. In part 1 of this series, we learned about cycling your aquarium, and that the end product of this cycle is nitrate. When nitrates build up to about 20 ppm, they pose a threat to your fish, which is one reason why regular large water changes are so important. However; in addition to nitrate, there are dissolved organic compounds which also build up in the aquarium over time. The more dissolved organic compounds there are in the aquarium water, the more harmful opportunistic bacteria can proliferate. The more of these bacteria that are present, the more likely it is that your fish will get sick. Thus, water changes are the single most important thing you can do to keep your fish healthy and thriving!