There are 2 fish stores really close to my house. One is awesome. Great people who know what they are talking about and can and do give you great advice, and one where they tell you you can keep goldfish or a betta in a tiny little bowl, and that fish won't outgrow their aquarium, and where some people who work there don't even know you need air in the bag when you take your fish home. Seriously. I went to get some guppies there and the girl I talked to couldn't tell the difference between a male and a female guppy, which I can understand fine, but she didn't even put any air into the bag! Anyway, when I was younger, I went to a party where the "loot bag" was a goldfish. Pretty cool huh? I didn't know much about fish at all, but I did like them, and I was excited! It was a little goldfish in a bowl. It was a tiny bowl, so I bought it a bigger one (still less than a gallon.) while I was at the fish store, I thought I would get a couple more fish.
Okay. Before I go on, in my defense, I was a kid who didn't know that a fish should have a filter and a real tank. I also didn't know goldfish were so dirty. AND I was in a fish store where the people who worked there either didn't know either, or were happy as long as they were selling me their fish.
Anyway, I asked the girl there, and she said that goldfish and bettas are fine in a little bowl with no filter or heater. (I know goldfish are fine without a heater, but a betta should have one) So I ended up with 6 poor little goldfish in a tiny bowl. Needless to say, they didn't survive for a very long time. They all died within a month.
I didn't know what was wrong until later, when I got my betta. He lived a couple of years, but he was in that same little bowl. So yeah. Now I know not to have fish in a bowl with no filter. And certainly not that many!