When it comes to filtering an aquarium, there are three types or categories you will want to cover. Chemical filtration such as carbon or GFO media, mechanical filtration like filter socks or skimmers, and finally biological filtration. Biological filtration provides stability and makes the water safely inhabitable for fish and corals, protecting them against toxicity from their own waste and it is all made possible thanks to beneficial bacteria. 

What Is Biological Filtration?

A biological filter is a little different in that it is not a piece of equipment rather it's the natural process by which aerobic and anaerobic bacteria break down decaying matter (fish waste, leftover food, etc.) turning it into ammonia, nitrite, nitrate, and finally nitrogen gas. What science refers to as the nitrogen cycle. In your aquarium, fish and other animals would be immediately poisoned (to death) from their own waste or other decaying matter without the presence of the nitrogen cycle or biological filter. 

The primary function of the nitrogen cycle is to neutralize toxic ammonia and nitrite.

"Beneficial bacteria" is a term aquarists often use to refer to the variety of different bacteria and microbes that play a critical role in the nitrogen cycle process. These bacteria will naturally grow and reproduce in your aquarium under the right conditions which we as hobbyists are responsible for providing. 

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Do I Need A Biological Filter?

Yes, home aquariums require successful biological filtration and the beauty is, biological filtration naturally grows or occurs in your tank becoming more robust and healthy over time. Beneficial bacteria grow on the surfaces inside your aquarium which include your sand, rocks, decorations, aquarium walls, and even filter equipment. Pretty much any surface that touches the water.

You can improve your biological filter by providing additional surface area for this bacteria to grow which is accomplished with biological filter media such as bio-balls or ceramic media like Brightwell Aquatics Xport. There really is a wide range of biological filter media options and all of them accomplish the same thing, provide more surface area for bacteria to grow. Some of them are simply better because they provide more surface area than others. 

It is important to also understand that beneficial bacteria take time to grow in your tank and exactly why we have to cycle our aquariums before adding fish. Technically speaking, aquariums are constantly cycling meaning beneficial bacteria is always growing and reproducing on the surfaces in your aquarium throughout the entire life of your tank. Populations will rise and fall based on available habitats (surface area) and nutrients in the aquarium.  

How To Setup A Biological Filter?

Patience is really the key to biological filtration. As your aquarium cycles, the beneficial bacteria will begin to grow. Over time, that bacteria establishes healthy populations becoming more diverse and robust. This is exactly why new aquariums are far more finicky or less stable than more mature aquariums. Tanks that have been running for 2 years tend to have a more diverse and stable biological filter.

The majority of your "biological filter" will occur in your sand bed and on your live rock. You can then provide additional biological filter media (more surface area) to improve the bio-filter in your tank. For tanks without sand or rock, it is all that more important to provide sufficient media for bacteria to thrive. In addition to media, there are additives you can dose to the tank that contain beneficial bacteria to help support the bio-filter. There is also a special process called "carbon-dosing" but that should only be reserved for more advanced hobbyists. 

So in theory, you don't have to do too much. Just provide the surface area for bacteria to grow in your tank, then just be patient and allow that bacteria to establish. As fish are added, waste is created that feeds the bacteria that create the nitrogen cycle. Technically speaking, you have a successful biological filter when your tank is cycled, when ammonia and nitrite are no longer present and nitrates begin to form. From that point forward, the bio-filter only becomes more diverse and robust. With that comes increased stability and the ability of your tank to successfully handle a bigger bioload.  

Types Of Biological Filter Media

Bacteria Based Additives

Bacteria additives can serve a few different purposes. Most commonly, they are dosed into new aquariums to help establish a successful biological filter. There is a wide variety of options for hobbyists but for the most part, they all do the same thing, at least anecdotal evidence suggests they do. Here are some of our favorites. 

There are also supplements that harness bacteria and can be dosed on a routine to target and break down detritus, helping to keep your tanks clean such as Brightwell MicroBacter-Clean and Dr. Tim's Waste Away. If algae is your problem, there are bacteria-based solutions that can help with that too such as Vibrant.

Carbon Dosing

For most hobbyists, carbon dosing isn't a part of the regular biological filter but it is important to cover based on its rising popularity among saltwater tank owners and the simple fact it harnesses bacteria. Carbon dosing is a process by nitrifying bacteria growth is accelerated in your aquarium via the use of special media (Biopellets) or additives (Red Sea NO3:PO4).  In turn, this helps to process and remove nitrates from the aquarium water. Really is only required for tanks with a heavy bioload and elevated nitrates are a constant battle or in situations in which you're trying to achieve an ultra-low nutrient reef aquarium (ULNS).

While carbon dosing can be very effective, modern filtration equipment has made it pretty easy to achieve a clean and healthy environment in a well-maintained aquarium. Furthermore, when not used correctly you can disrupt or even crash the biological foundation in your aquarium.