The common goldfish, and almost nothing about the way most people keep them is right. They are not a bowl fish. Given the room they reach 30cm or more and they will outlive a lot of the furniture in the house, twenty years and up with decent care.

Where they really come into their own is an outdoor pond. They are hardy enough to sit through a Taranaki winter, and a pond gives them the space they actually need. Indoors you are looking at a big tank and serious filtration, because they produce a lot of waste for their size. Keep them with other fast single tailed types like comets or shubunkins. Put them in with slow fancy goldfish and they will take the food before the fancies get near it.