Aquariums rely upon filtration to export nutrients and maintain a safe environment for their inhabitants. Without filtration, fish and other animals would quickly perish due to poor water quality; ammonia poisoning being the most likely culprit. We can categorize aquarium filtration into three different types of filtration, all of which work together to maintain suitable habitat.   

  • Mechanical Filtration
  • Chemical Filtration
  • Biological Filtration

The order in which the water passes through each stage of filtration is important to achieve the most efficient results.  This will become more evident as we discuss the exact role and capabilities of each stage of filtration.  

Mechanical Filtration

1. Mechanical Filtration

This is ideally the first type of filtration the water encounters when passing through a filtration system where waste particles are physically removed. This could be a filter sock, filter pad, fleece roller, or sponge that essentially sifts the water and traps debris. The mechanical filtration media (filter pads, filter socks, etc.) is removed and replaced or cleaned on a frequent basis as a means of exporting the trapped waste. This would also include protein skimmers that physically remove hydrophilic waste particles via the process of foam fractionization.

Because mechanical filtration traps debris and is the first stage of filtration, it clogs up and requires more frequent replacement or cleaning. Filter socks or filter pads should be replaced every few days and protein skimmer cups should be cleaned and emptied on a similar schedule for the best results.

Chemical Filtration

2. Chemical Filtration

By definition, this is the process where contaminants are removed via a chemical reaction of some kind. As it pertains to aquariums, we are generally referring to chemical filtration media such as carbon, GFO, or synthetic polymers that adsorb particular dissolved contaminants from the water. 

For example, GFO or granular ferric oxide targets and removes only phosphate and silicate from the aquarium water. Carbon is more of a broad range chemical filter media that will remove tannins, phenols, chlorine, and chemical contaminants and ultimately improve water clarity, eliminate odors, and remove toxins. Purigen is a synthetic polymer that acts very similar to carbon and is designed to remove organic waste. 

Chemical filter media will become exhausted over time meaning it can no longer adsorb any more contaminants. The contaminants are trapped within the media and are effectively exported from the aquarium when the media is removed and replaced with new media.

Biological Filter

3. Biological Filtration

Biological filtration is the process where some biological process aids filtration by removing or processing organic waste. Nitrifying bacteria are the primary source of biological filtration and are an essential part of ALL aquariums to eliminate the presence of toxic ammonia (fish poo). These beneficial bacteria sustain the natural nitrogen cycle in your aquarium and will grow on all of the surfaces of your aquarium including the tank walls, submerged equipment, and biological filter media. Porous live rock, sand or gravel, ceramic rings, sintered glass, and plastic bio-balls are all various types of bio-media that increase the available surface area for nitrifying bacteria to colonize. 

  • More surface area = more beneficial bacteria = ability to process more ammonia

Additionally, bacteria can be leveraged using bio-pellets, carbon dosing, and the Zeovit Method which are all forms of biological filtration that reduce nutrient (nitrate and phosphate) levels in the aquarium. 

Refugiums, algae scrubbers, and macroalgae reactors are also categorized as biological filtration as well because they rely upon the growth of algae to remove excess nutrients which is very much a biological process. The algae sequester nutrients (nitrate and phosphate) as it grows and locks those nutrients up in its tissue. The algae are then harvested from the aquarium, carrying all those locked-up nutrients with it. These algae-based filters also provide prime habitat for beneficial microorganisms like copepods and amphipods to proliferate. The tiny crustaceans help keep the aquarium clean by consuming nuisance algae and detritus in addition to being a great natural food source for the fish and other animals in your aquarium. 

While all of these methods fall under the "Biological Filtration" umbrella, each works in its own way to benefit the aquarium. 

  • Biomedia: Harbors nitrifying bacteria which eliminates ammonia and creates nitrates.
  • Biopellets, carbon dosing, Zeovit: Removes or eliminates nitrate and phosphate; supplies nutrient-rich biomass as food for corals.
  • Algae filters: Removes nitrate and phosphate and supports healthy biodiversity; supports a natural food chain for carnivorous fish 

Filter Media Order

The Importance Of Order In Your Filtration

We numbered the filtration types because this is the most effective way to organize the various types of filtration.  When passing through the filtration, the water should first be cleaned of physical debris via mechanical filtration so it does not wind up clogging the succeeding chemical and biological media. Clogged-up media simply doesn't perform effectively so the goal is to remove a majority of that larger debris before it can get trapped in subsequent stages of filtration. 

Chemical filtration often targets specific contaminants, down to the molecular level, that are simply too small for mechanical filters to remove. Chemical filter media is not always required to maintain a healthy aquarium but can be used as a very effective tool to maintain clarity, remove odors, and correct water quality issues.  

Finally, the water encounters the biological media where bacteria can take over and process any ammonia. Nitrifying bacteria process toxic ammonia and produce nitrate which is not directly toxic to the fish. Technically speaking, biological filtration is happening throughout the entire aquarium and when using bio-media specifically, you want to ensure it is always placed somewhere after the mechanical filter to reduce that clogging effect we discussed above. A stable population of nitrifying bacteria is 100% mandatory and essential for all aquariums to survive. 

The additional types of biofiltration including refugiums, algae filters, biopellets, etc. are optional and are most valuable for their ability to reduce nitrates and phosphates.