Goldfish are one of the most popular pet fish in the USA. They come in many color and shape varieties and they are very hardy, which makes them a great choice for beginners. Goldfish do not require heat, filtration or any kind of special care (though they do great in heated, filtered tanks too). You can find them in black, white, gold, tri-color, brown, and any combination of these colors. Prices (at the store I work at) range from 35-cents for a comet (feeder) goldfish, to $13.99 for a pom-pom goldfish or a pearlscale goldfish.
Despite their hardiness, there are downsides to goldfish as well.
1. They tend to get larger than most people think. When you see a cute 1-2 inch fish at the pet store, you don't expect it to grow to 8-14 inches! While they may look lovely in a bowl for the first few months, if you want a healthy fish you will have to give it more space. For a single adult comet goldfish (those 35-cent feeders I mentioned before) you should have at least a ten gallon aquarium. The general rule of thumb is 1-inch of fish per gallon of water. If you think your fish needs a friend, you should be prepared to set up a 20-gallon (or larger) aquarium. A commonly circulated myth is that goldfish only grow as big as their tank - this is not true! A goldfish will outgrow a bowl in 6-12 months or less! If you don't get him a bigger space to live in, chances are he will die.
2. Goldfish are dirty. Goldfish are a member of the carp family. They are messy on all levels. They spike the nitrates in your tank, which is the nice way of saying they pee/poop more than other fish. You'll find that to keep your tank from quickly becoming toxic, you have to clean it far more often than you would with other fish. You'll need filtration about double what your tank size (20 gallon filter for a 10 gallon tank, 40 gallon filter for a 20 gallon tank, etc). Don't forget to change the filters regularly.
3. You can't really mix them with other fish. Unless you have a huge tank (55-gallon or bigger) and/or you clean your tank on a weekly basis, you're really limited to other types of goldfish to add to your tank. As I mentioned, goldfish spike the nitrates. While goldfish aren't as effected by dirty water with high nitrates (since they put them there), other fish quickly die from it. The other problem is that goldfish are likely to eat any tank-mates that they can fit in their mouths, and when you have an 8-14 inch fish, a lot of commonly kept community fish become incompatible.
4. Goldfish are always hungry. Many a goldfish has died because unknowing owners continued to feed them when the "looked hungry," nibbled on rocks at the bottom of the tank or "begged" for food at the surface. Goldfish will eat just about anything they can put in their mouths. They taste everything, sometimes again and again and again. They'll pick up and spit out the rocks at the bottom of your tank, they'll eat any live plants you add to the tank, they'll eat smaller tank-mates if they can easily catch them and fit them into their mouths, and yes, they will eat and eat and eat until their stomach literally explodes in their body and they die. If you accidentally put too much food in the tank, it needs to be removed and cleaned up immediately!
5. Goldfish can live a long time when properly cared for. Most people expect a goldfish to live a year or two. If given appropriate space, regularly cleaned water, and a pinch of food every day or two, they can actually live 10 years or more!
If you are considering starting in the world of aquarium fish, goldfish are great. just keep in mind that they grow, they're dirty, you shouldn't mix them with other fish, and you could be making an 8-12+ year commitment. While they're cheap and readily available, please do your research before buying one!