Goldfish come in a wide variety of types that may have slightly different needs and given the proper environment, mature to different sizes.

I keep the type most common in Japan, called Wakin, that are bred to be pond fish best viewed from above, like Koi. Wakin mature to 18 inches and have been know to go to 24. They are are somewhat rare in the US.

On the other hand, an Oranda that reached 12 inches in body length would be huge for it's variety but would also be at least twice as large in diameter than a full sized Wakin, or Pond Comet, which is really only a Common Goldfish, bred for intense red color and longer fins than the Commons.

Given the choice, it's logical to assume that most varieties would be happier in a pond simply for the room it afforded and excepting the extreme fancy types that need warmer water, with just a small air hole, most varieties of Goldfish can and do drop into a torpid state and winter under ice.

I have never seen any Goldfish exibit any habits odd to Goldfish. Each species of fish has it's own habits but being so totally domesticated, Goldfish can become quite confiding with their keepers and can can be trained to perform simple tricks, such as swimming through hoops on command.

Due to the fact that they are primarially cold water fish and need a lot of dissolved oxygen in their water, coupled with their sheer bulk, no Goldfish should be kept in a tank less than about 4 - 5 gallons per body (not including tail) inch of fish, with either very strong filtration, or daily water changes. As they grow larger, the need for a larger tank increases expotentially.

There are a number of websites devoted to the keeping and breeding of Goldfish. Google "Goldfish" and you will find a lot more detailed information.