Filters for aquariums come in a variety of shapes, types and sizes, but they perform the same basic functions, and that is to circulate the water through a form of fractionation, straining, or cleansing medium at such a rate that the water is kept clean physically and biologically by breaking down waste products, including: nutrient proteins.

The three major categories are mechanical, biological and chemical.

• Mechanical filters remove unwanted materials and particulates from the waters column by straining the water through a type of absorbent material, such as filter floss.
  
  
• Biological filtration involves bacteria and other microorganisms, which converts your fish's waste into less toxic substances through a process call the Aquarium Nitrogen Cycle. Biological filters will convert your fish's toxic waste ammonia into nitrite, then from nitrite to nitrate, which is relatively harmless.
  
  
• Chemical filtration as the name implies can remove toxins and other chemically induced material from your aquarium system with the use of activated carbon. It's necessary to change activated carbon, after it's exhausted. Some filtration apparatus implement more than one category, for example Aquarium Protein Skimmers can use both mechanical and biological processes to filter the water.