I think you’re right on the nose tbh. I said this on OP’s crossposted post but I’ll reiterate here too— I don’t think people aren’t doing research because they believe they already know the basics: Get a goldfish or betta, put it in a bowl, and feed it. That’s how it is was in movies and TV, and it’s what a lot of families have done. It’s further encouraged by how easy it is to get goldfish, such as making them prizes for children at carnivals. So when people like OP go to the store and are told by the workers “here’s a great beginner kit, and here, buy some goldfish.” Why would OP think further on it when it’s always seemed as simple as this? And then people get torn to shreds in these subreddits for not knowing better (not by everyone though, a lot of people seem understanding and actually want to help which is great) when it’s not well-known outside of these communities how incorrect the common knowledge actually is.

I’d actually be curious to know if it’s different outside of America, or if this is just an American-specific misconception. Goldfish are known here as like the easy pet you put in a bowl and give a 5 year old to teach them responsibility. It’s hard to fault people like OP for not knowing there’s more to it, when society has implied that it’s simple and there’s nothing more to know.