Updated: Jun 6, 2022
The three forms of filtration in your tank are mechanical, chemical and biological. In my opinion, biological filtration is the most important. This type of filtration is responsible for the conversion of toxic nutrients into less toxic organic by-products. If done correctly, biological filtration goes a long way toward a self-sustaining system.
Biological filtration comes in the form of live plants and bacteria. Live plants absorb excess nutrients and certain types of bacteria do the same. Let's discuss which plants and bacteria do this best, where they do it, and how to provide an ideal habitat for them to grow and do their thing.
Live Plants as Biological Filtration
Live plants in freshwater and macro algae refugia in saltwater are a go to for me and my aquariums. There are varying levels of intensity when it comes to plants and macroalgae. The simplest is to provide nutrients (nitrogen, phosphorous, potassium, iron, trace elements), some of which come from your fish waste, and light. For more difficult freshwater plants, add a CO2 system. If you provide enough light and carbon, the plants will suck nitrogen and phosphorous in the form of nitrates and phosphates out of your tank with impressive efficiency, leaving none for nuisance alga. As nitrates can become toxic and phosphates contribute heavily to algae growth, live plants and macro algae can go a long way in providing excellent biological filtration.
Beneficial Bacteria in the Aquarium
As for bacteria, their are two main types we are concerned with. The first is nitrogen fixing aerobic (oxygen requiring) bacteria. These species convert compounds containing nitrogen into useful forms of nitrogen plants and macroalgae can use. They include the Nitrosomonas and Nitrobacter genera.
The second are various species of anaerobic (non-oxygen requiring) bacteria. These types breakdown end-stage nitrogen compounds (nitrates) into simpler, less toxic, compounds which can be reused by the aquarium.
The ideal habitat for nitrogen fixing aerobic bacteria is any where in your aquarium where oxygen is present and water is flowing past. Anaerobic bacteria require a low oxygen environment, this usually means slow moving or still water. Still water in the aquarium is not ideal, so how do we accomplish this?
Biological filter media is a general term that applies to any type of porous material added to your filter to house mostly nitrifying bacteria. The media should be porous to provide as much surface area as possible for bacteria to grow on. The best type of biological media provides habitat for both aerobic and anaerobic bacteria. One example is the ceramic media from CerMedia. (Did not receive any money for this, just a fan). What makes this type of media the most efficient is the outside is open enough to provide good flow and high oxygen for aerobic bacteria, while the inside center is compact and restricts flow, providing a low oxygen environment for anaerobic bacteria. The best of both worlds in one media. As with all biological filtration, it is important never to wash or rinse your media. The bacteria colonies that live in the interstitial spaces should remain established. If your media becomes clogged or dirty for some reason a quick shake or rinse in old tank water can help as it does not contain chlorine or antibiotic chemicals.
Biological Aquarium Filtration
The whole purpose of biological filtration is to remove organic compounds before they build up to toxic levels. We can do this mechanically with a water change, but why not let nature do the work for you? The other purpose of a water change is to add certain elements back into the aquarium, so don't forget to dose your trace elements if water changes become less necessary for your tank. This is much more complicated for saltwater systems and it is hard to beat the comprehensive list of elements in a good salt mix. Still, dosing the elements that are used faster before your next water change is always a good idea.
Check out these articles on ideal water parameters.
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