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All tanks need some form of filtration, whether it be biological, chemical, or mechanical in nature, to prevent waste from turning into harmful chemicals in our tanks.

Most reefers employ a combination of the different types of the above. A common method is the Berlin Method, which basically utilises live rock for biological filtration and a protein skimmer to remove waste.

Its very important to understand the nitrogen cycle when attempting to understand filtration

The Tank Journals are a good start for what is possible and what people are doing currently.

It is recommended to look into the different types available and see what meet your needs. Commonly, the Berlin method is the minimal approach followed.


Biological filtration is probably the most important filtration in a marine aquarium and it is possible to have a successful aquarium with only this type of filtration. It is called biological filtration because it uses organisms to either extract compounds from the water or to convert it to something less toxic or more easily removed.

Most biological filters utilise bacteria to do the filtration. An active biological filter of this type supports the nitrogen cycle.

There are a number of different filters that are classed as biological and they all essentially do the same thing which is to provide a environment for nitrifying bacteria (sufficient surface area to colonise and sufficient oxygen) and in some cases denitrifying bacteria (sufficient surface area to colonise and low or no oxygen).

The more surface area you provide in the aquarium, the larger the population of bacteria that is possible for biological filtration. It may be tempting to just keep adding different forms of biological filtration to maximise the filtration capabilities or as some kind of backup. In reality, it is generally not needed. An aquarium with a sand bed and live rock will have more then enough surface area such that other forms such as a trickle filter or canister filter are not even unnecessary, but may also be detrimental.

Algae and plants are another form of biological filtration. Here the algae or plants absorb compounds from the water such as ammonia, nitrate and phosphate. Macro algae (often in a refugium) is the most common, but an algal turf scrubber can also be used. Plants that can be utilised for biological filtration include sea grasses and mangroves.

Forms/types of biological filtration:


With chemical filtration, compounds are removed from the water using chemical (non-organic) means. They utilise the chemical properties of other compounds to take up undesired compounds. Activated carbon is one of the most common forms of chemical filtration, but there are also resins and even natural occurring minerals (such as zeolite) that are capable absorbing compounds such as phosphate.

Forms/types of chemical filtration:


Mechanical filtration involves the physical removal of matter from the tank. Filter wool is probably the most obvious form of mechanical filtration. Protein skimmers are also a mechanical form of filtration, although they make use of the chemical properties of complex molecules such as protein.

While protein skimmers are generally encouraged in a marine aquarium because they can remove a large amount of organic matter before it enters the nitrogen cycle, other forms of mechanical filtration are not recommended due to the tendency to trap organic matter which can the break down if the media are not cleaned regularly.

Forms/types of mechanical filtration: