Installing a good filtration system is the most important part of the entire fish room, assuming it is heated to a stable tropical temperature and assuming your tank starts out with the right chemical environment (moderately hard water at neutral pH, and tolerable levels of other toxic chemicals or metals). Filtering equipment traps debris and houses colonies of beneficial bacteria. It is the key element in supporting the biological cycle in an established aquarium.
There are three types of filtration: mechanical, biological and chemical. Mechanical filtration traps particles of waste, debris and uneaten food. Biological filtration converts toxic chemicals produced as waste by your fish into less toxic substances. Most notably, toxic ammonia is converted to less toxic nitrite, and nitrite to nitrate.
The filter material traps debris and provides a rich surface
for bacterial colonies. The round ceramic filters provide
maximum surface area, and are dark in the interior,
a condition favored by bacteria.
Chemical filtration uses chemicals (such as dechlorinators and carbon charcoal) to purify the water. When you dechlorinate tap water before filling your tank, you are practicing chemical filtration. In fact, if you set up a holding tank and aerate water for a day or two, you do not need to add chemicals to your tank, other than chemicals to add hardness or alkalinity to your water. Charcoal is totally unnecessary for reasons we provide elsewhere.
Partial water changes often accomplish what chemical additives sold to the hobbyist claim to accomplish. If you have medicated a tank and destroyed or reduced the effectiveness of the biological cycle, adding in chemicals that claim to jump start the biological cycle may cause more harm than good. They may lull you into a sense of security and you return your fish to same feeding levels as before. (The jury is still out on the effectiveness of such products.) Putting water into the tank from a healthy tank is usually much more effective. Partial water changes over time dilute toxins and remove waste. In the end, putting a filter into the tank from a healthy tank is the most effective way to re-establish the biological cycle. (However, you should bleach the filter from the medicated tank as a safety measure.)
Human beings have advanced a long way from living in caves, partly because they have learned technology can make them more comfortable and give them more leisure. Product manufacturers have learned to cater to this. Those manufacturers who offer a chemical or technological solution alternative to more labor intensive ways of maintaining a tank (changing water and cleaning the tank and filters) are making powerful appeals to our faith in technology. But it should be said emphatically that the technological solution is often a devil’s pact, where you will pay for the freedom and comfort you enjoy know, later. Fifty years of technological progress in the fish room have not superseded the oldest and most reliable form of mechanical and biological filtration available to the guppy breeder: the cheap plastic corner filter stuffed with filter media and attached to an old-fashioned air pump.
Changing water and cleaning the filter when it needs it (every two or three weeks depending on the bioload of the tank) are still the best and most reliable contributions a hobbyist can make to an ongoing healthy environment for the guppies.
There have been advances in our knowledge of the biological cycle. For example, most bacterial systems today are made out of black plastic because we have learned that the bacteria that contribute to the biological cycle grow best in darkness. A more thorough discussion of the biological cycle appears in a section on this site devoted to it.