First, congratulations on your new pet! Fish keeping can be a very rewarding hobby for both children and adults alike.
Now that I've lured you in with a cheerful welcome, let me tell you that taking care of your little goldfish is probably more work than you thought. Sorry, but you can't just stick it in a glass bowl and drop in a few flakes once a day.
It seems like that's all you should do, though, doesn't it? Anyway, it can still be entertaining to keep goldfish, so here are some tips to keep your new friend happy and healthy. (This article is intended for beginner goldfish keepers.)
First of all, you can be about 110% sure that those fish in the little glass jars are what's known as "feeder fish."
They're the goldfish you see housed by the hundreds in pet stores, and only cost about 10 cents each (including the pet store markup!).
Nonetheless, it is a beautiful living creature that needs you to give it a good home and take care of it. So while I want to make sure you're aware of the cost vs. value issue in this situation, I'm going to focus on making the most out of this experience for both you and your fish.
The one thing to keep in mind about your fish is that, as a fish intended to feed other animals, it probably hasn't been treated too well in its short life. Many of these fish have internal parasites or other diseases due to poor care and stress. But this doesn't always mean the worst. Goldfish are also one of the hardiest fish you'll ever find, and with proper care, your little fishy may do just fine.
If you actually did get your goldfish at the carnival, it was probably handed to you in a clear plastic bag with little to nothing else.
Which means, unless you were quick to get to the pet store, it's probably sitting in some sort of Tupperware container, or maybe even an empty butter tub. Don't worry, it's nothing to be ashamed of. But let's get something a little bit more suitable. Some people (like carnival workers) might tell you that a little glass or plastic fish bowl will last throughout your fish's entire life. And that's true, because your fish will only live about a month in something like that.
What you're going to want to get is a good glass aquarium, and you're in luck, because it seems full aquarium packages are available just about anywhere nowadays for $30 or less.
This is a good price, and if you're still thinking about a plastic bowl, consider that an aquarium can last for decades and can be resold if you wish. So head to your pet store, department store, or maybe even drug or grocery store and pick one up. A ten gallon is the old standard, and will work perfectly for up to 5 goldfish. Included in your package should be the aquarium, a lid (if the lid holds a light, make sure there is something to keep condensation away from the bulb), a filter and pump system, and possibly some gravel and plants for decoration.
The obvious is fish food. Goldfish do fine on regular old fish flakes. You can try the fancier treat foods if you like, but chances are they were raised on the cheapest stuff available and that's what they'll like. So compromise and buy some good quality fish flakes from a recognized brand, like TetraFin.
How's your water? If your tap water is chlorinated by the city, then you have two options: Everytime you change or add water to your fish tank, you let it sit in an open container overnight before your fish come in contact with it... or... buy some dechlorintator. The second option is the easiest, and also can provide some healthy benefits for your fish, as well. It's up to you, but a good dechlorinator like Stress Coat will defintely benefit your fish. What happens if you use straight tap water? It's an ugly story involving gill lesions and suffocation. Let's not think about it. Lastly, maybe an algae eater. They'll just help keep things a little bit cleaner.
Take the fish tank out of the box and take a look at all the parts. You'll first want to give the inside of tank (and anything that will eventually go in the tank) a good rinse off. Then go ahead and attach your pump and filter according to the directions included with your kit. This varies depending on the type of filter system your aquarium uses, but should be quite easy with some simple instructions.
If you haven't already, decide where you're going to put this aquarium. You'll need a level surface with an electrical outlet, not too close to a window or any other heat source. A place in your home that has a lot of traffic would be best; out of sight, out of mind, remember? That's not a good thing for a pet.
Once you've got your aquarium up and running (with a dechlorinator in the water, remember!), then you'll want to get ready to put your fish in the water. If you still have your fish in the plastic bag (or any other fish you may have just bought), allow the bag to float on top of the water in your aquarium for at least 10 minutes or so. This will allow your fish to adjust to the temperature difference before being plunged right into the water. If you can't do this, however, I try to tilt the bowl the fish is in on its side as I lower it into the water. If you do this slowly enough, the bowl water and aquarium water will begin to mix a little at a time, and your fish will eventually swim into the aquarium once it's adjusted to the water.
After you have put your fish in their new home, sprinkle a few flakes of fish food on the surface. Hopefully your fish will notice these and eat them, and it's my belief that this will help calm your fish down and feel more comfortable in this new environment. (Hey, if they're feeding me, it can't be that bad!)
Now, the hard part is done and you can sit back and enjoy your aquarium. Remember to feed your fish small amounts of food two or three times per day, think about two flakes per fish each feeding. You'll want to remove about 20% of the water from your aquarium once every two weeks, and replace it with fresh, dechlorinated water of the same temperature. Change the filter (if applicable) according to the manufacturer's instructions. You also may want to have your water periodically tested (especially if your fish appear ill) for chemical imbalances. Many petstores that sell fish will do nitrate/nitrite and pH testing for free, and can offer suggestions to just about any problem you may run in to. Good luck!