The fish tank filter is the most important component of a successful aquarium setup. Filters are what keep the water from becoming stagnant and lifeless. Filters have three purposes. They are the breeding ground for the special bacteria that work with the chemicals produced by fish waste. This is crucial for the biological cycle of your tank. Filters also remove the debris from the water, and finally, some filters allow you to place active carbon and other chemicals when they are needed for the tank.
There are different kinds of filters: you have external and internal filters, and either type can be used in saltwater or freshwater aquariums.
In the category of internal filters, you have the sump tank placed below the main tank. This kind of filter is used for saltwater tanks. It works like this: the water from the fish tank spills into an overflow, which has a pipe leading to the sump. The sump contains filter media and a pump, which sucks the water in and then pushes it through a tube back into the main tank. The disadvantage of this kind of filter is that it can be expensive and it can also be noisy.
There are also the under gravel filters that are normally used in freshwater tanks. An under gravel filter is a raised grate on the bottom of the tank, under the gravel or any other substrate you want to use. This grate has two holes where you will attach a pair of vertical plastic tubes. These tubes work with either a powerhead or an air stone inside them. The way it works is that the water is forced to flow through the gravel to go under the grate, where it will go up the tube and into the tank. The gravel itself works as the filter media. It collects leftover food and waste.
The preferred kind of filter is the external kind. There is the kind that you can hang from the back of the tank, and the kind that goes underneath, in the stand. All the filtering happens there. The water gets sucked out into the filter, passes through the filter media, and goes back into the tank. This type of filter can be expensive, but it is also the most effective and low maintenance. You just need to make sure that the filter is large enough for your tank size. It has to be able to circulate ten times the volume of water per hour. A 75-gallon fish tank needs a filter that can circulate 750 gallons per hour.
Most of these filters can be readily found in stores and online, but the more expensive external filters, since they need custom-made tanks, are harder to find in a general pet store. It might be worth it to save that money and get a good-quality external filter. Check this area for really cheap fish tank filters.