A biological filter is basically a smaller tank under the actual aquarium. In this 'filter' tank the water goes through several compartments with filtering substrate, where large colonies of cleaning bacteria will develop. Besides the positive effect of the bacteria a biological filter larges the overall volume of the water, which makes the water quality more stable. The effect bacteria have on the water quality is not limited to a biological filter, all filters will contain the positive bacteria over time, but because of its larger size the effect is just better.

There are two other kind of filters often used in our modern aquariums, for smaller tank sizes there are often small inside filters used. They have a rather small capacity and can over time become a problem because the waste products are remaining inside the water. I would only recommend these inside filters for temporary usage, like for a breeding tank or a quarantine tank.

The most used filter is an external one. If you buy one with enough overcapacity it is a good filtering method. Be sure to clean the filter substrate every two months by washing it out with water of the same temperature ( more or less ) as the aquarium water. This way you will get rid of the waste products without loosing all the bacteria.