Goldfish is one of the most popular fish because of their high availability at petshops. They are in fact not good beginner fish because of the high amount of waste they produce. Without an established biological filtration as in the goldfish bowl, they need a lot of water change otherwise the water quality quickly deteriotes.
One of the interesting thing about goldfish is that unlike other fish, some goldfish are bred to be viewed from the top. Ie ranchus.
Note that goldfish does not stop eating even when they are full, making them prone to die from bloat or overfeeding.