Filtration and Aeration
There are several types of filters, including sponge, undergravel, and hang-on outside. The sponge filter (basically, a piece of foam sponge that is fitted with a lift tube) is inexpensive, easy to maintain, and quite efficient. It is an inside filter, meaning it must be situated within the tank and cannot be hung on the outside like some other filters. With some effort, a sponge filter can be hidden by rockwork or decorations, but it will never be unnoticeable. They tend to be most useful for tanks under 20 gallons in size, which are not recommended for beginner hobbyists.
The undergravel filter is made up of one or more slotted filter plates that cover the bottom of the tank with a small amount of space left beneath them. Lift tubes are fitted into the filter plates and the plates are covered with several inches of gravel. Water is drawn down through the gravel, into the space below, and then pulled up the lift tube and back into the tank. The downside of undergravel filters is the upkeep. By performing weekly deep vacuuming during water changes, many problems can be avoided, but if debris is allowed to build up under the filter, an extensive cleaning will need to be performed, including the dismantling of the entire tank.
The hang-on outside filter is highly recommended for novice aquarists. Most of these filters use a water pump to bring water from the tank up into the filter, where it moves through areas of filter media and, with the help of gravity, returns to the tank. There is no such thing as over-filtration, only under-filtration, so buy the biggest and most powerful filter that you can hang on your tank.