While you may be enticed to take that goldfish offered at a fair or carnival as a prize, thinking you can just get a little bowl for it on you way home, you might want to entertain a second thought before you do this.
Goldfish, common (or comets) and fancy, are a rapidly growing species of carp that will need a tank much larger than the average goldfish bowl. Common goldfish (comets) can live up to 25 years when properly maintained and can grow to around 16 inches in length rather rapidly, about six to eight inches a year. Fancy goldfish can live up to 15 years and grow to sizes of up to 12 inches, also rather quickly.
As a species of the Carp family, goldfish are a bottom feeding omnivore. They will eat anything, including other smaller fish and live shrimp. Though this is not a recommended diet, but should be thought known if contemplating putting them in with other fish. Goldfish are not really compatible tank mates with any other freshwater tropicals, as they are a cooler water fish, preferring room temp water between 58 and 65. Where as most other tropicals (hence the name) prefer warmer water of 70 + degree water. Since goldfish are a bottom feeder they are abundant waste producers, causing tanks to get dirty quickly.
Tank requirements for a goldfish will probably shock most people who have images of little gold fish swimming around a bowl. A tank to house a single full grown common goldfish, if following the 1 inch per gallon rule, would need at least 160 gallon tank! A single fancy goldfish could be housed in a 150 gallon tank comfortably into old age, but it has to be the only fish in the tank. This is why most goldfish owners have either large ponds to house their goldfish and koi, or for those where money is no object, extremely large 1000 gallon wall tanks with many fish in their homes.
An overcrowded goldfish tank will result in goldfish that are sick and that won't live happily to their full potential. Keeping them in small overcrowded tanks is a cruel death sentence for many fish, that the owners aren't even aware of. The tank care of a single 150 gallon tank with one full grown fish will require a very good carbon filter, under gravel filter, gravel vacuuming weekly, and sometimes up to 50% water changes weekly.
The water in a goldfish tank has to be carefully monitored for ammonia spikes, nitrite and nitrate spikes, and dirtiness.
If you can not keep a large tank like that, you might want to consider a smaller 10 gallon tank with small tropical fish, or if you like the look of an unfiltered bowl type set up, invest in a well bred Betta Splenden (Siamese Fighting Fish.) The Betta fish is easy to care for and there are many interesting Betta tanks that much more aesthetically pleasing than a common goldfish bowl.
So in conclusion, I beg you to rethink your decision to win or purchase a goldfish and stick it in a bowl. Placing a goldfish in a "goldfish bowl" is giving it a death sentence. Goldfish are not disposable pets, but are living creatures that deserve to be cared for properly. Most pet owners wouldn't buy a dog and expect it to live in a crate and actually thrive. So please have consideration for your goldfish. Give it a big tank, with plenty of room, plenty of aeration, plenty of filtration, and realize that it will get very big and live for a very long time.