Fish like clean water. Admittedly many can survive quite dirty conditions, for example goldfish can tolerate mucky unfiltered fishponds for years, but the fact is they would be happier in cleaner water! Dirty water increases the likelihood of disease and for long term health and happiness your tank or pond should have a filter. Most tropical and all marine species are nowhere near as tough as goldfish, and if you do not keep the water clean with a filter they will die!
The purpose of filtration is to degrade biological waste (fish waste, uneaten food, decaying plant matter), chemical polutants (chemicals introduced with tapwater, houshold cleaning and smoking), and mechanical filtration to keep the water looking clean.
Biological waste: this is the most important part of filtration. Bacteria is used to convert toxic chemicals into safer ones, and ultimately into plant fertilizer. This is exactly the same way that rivers, lakes, seas and oceans keep themselves clean. The bacteria (known as nitrifying bacteria) lives within the filter, but takes several weeks to become established. Bacteria can be added, or will appear naturally but as it takes so long, your aquarium should be set up for several weeks before reaching its final stocking density.
The process in which bacteria converts Ammonia (fish waste) into Nitrates (plant food) is called the nitrogen cycle, and although the chemistry may appear complicated, it is essential that fishkeepers understand the basic principles.
Chemical polutants: the best way to remove chemical pollutants (rather than biological) in an aquarium is by the addition of activated carbon. Carbon acts as a sponge that soaks up all the dangerous chemicals. It is not essential to use carbon; if you are cautious and treat the water going into your aquarium correctly there will be no chemicals to remove, however many aquarists use it just in case! Carbon can be added to canister and box filters.
Mechanical filtration: filter floss is cheap and effective. Larger particles in the water are filtered out keeping the water clear, clean and looking its best. Filter floss can be added to canister and box filters.
Types of filter
Well planted aquariums are not only great looking, they are also very beneficial to the fish. They oxygenate the water, absorb nitrates, reduce algae and provide refuge for fry (and other timid species). Plants like Java fern, hornwort, Amazon sword, Vallisneria (eel grass), Cryptocoryne and Anubias can all do well in a typical home aquarium with the smallest amount of attention