Keeping fish is not particularly hard, just different than keeping most other kind of pet because they live in water. Fish either come from freshwater or saltwater but there are a few varieties that can live in either.
Besides adding an interesting focal point to any room an aquarium has been consistently singled out for its ability to lower stress levels. If you are considering setting up an aquarium you will have a lot of company as recent statistics estimate that more than 1 in 10 households in the U.S. have at least one aquarium.
Being as fish spend all of their time in water that is where their waste ends up also. The key point in keeping fish alive is managing the waste they produce that will pollute the tank. A well-known quote about how to handle pollution is "The solution to pollution is dilution". For the fish keeper this translates to some common advice on what size fish tank is best which is "the bigger the better".
A freshwater tank is usually recommended for beginners and is by far the more common overall, in fact about 17 times more households keep freshwater fish than those that keep marine. There are a couple of reasons for this, cost and the more complex nature of maintaining the chemistry of artificial seawater in the aquarium. However the equipment available today makes successfully maintaining a marine tank easy even for beginners.
Fish are only part of the picture though plants and invertebrates often share the tank adding new dimensions to the underwater community. Crabs, shrimp, snails and crayfish are available for both freshwater and marine tanks. Marine tanks can also be set up as a reef tank that includes corals, anemones and other interesting inverts for a real 7 seas experience.
Initially setting up any aquarium takes patience as it naturally takes a couple of months for the biological processes in the tank to reach full equilibrium. During this time the aquarium is said to be "cycling". Slowly adding fish, following a very light feeding schedule and a few extra water changes is normally all that is required to avoid almost all of the problems that confront a new aquarist.
For those aquarists that find patience the hardest thing to bring to the hobby the timeframe can be shortened by using commercial products designed to speed things up. Using these requires some closer monitoring as you coax Mother Nature to speed up the process of cycling the tank. This is best attempted by more experienced aquarist but even new comers can have a good chance of success if they can get advice from someone who is familiar with how an aquarium works.
Once setup taking care of the tank usually consist of daily feeding and weekly cleaning including a 25% water change. A freshwater tank can be completely setup starting at under $10 a gallon and a fish only salt water tank for about twice that amount. Good luck with your fish!