Biological Filtration


Biological filtration is the removal of harmful waste chemicals that is produced by fish waste and uneaten food in the aquarium. These waste products causes the rise of ammonia in the water. Ammonia is very toxic in high levels and it is a common cause of fish deaths in new aquarists who fail to keep ammonia levels in check.


For biological Filtration to be set up we need

The main thing in bio filter is the beneficial bacteria. Without the beneficial bacteria, there is nothing to carry out the bio filtration.

The process where the beneficial bacteria consume chemical waste and converts them into less harmful chemicals while multiplying in numbers to establish themselves is called cycling. When the bacteria has grown to numbers sufficient to convert all excess chemical waste so that ammonia levels and nitrite levels is zero, we say the tank has cycled.

Learn about aquarium cycling here.


Flow rate and Beneficial Bacteria

The main consideration of flow rate for beneficial bacteria is that your pump should be able to deliver the water to them fast enough. The higher the flow rate is, the better it will be. A a general guildline, the flow rate should be big enough to cycle the water through the biological media 4-5 times per hour.

There is some rumours that a high flow rate can impact on the ability of the bacteria to process the nitrogenous waste. However, it is not a concern because assuming that beneficial bacteria cannot do well in a high flow area, they can still live on the inside surfaces of porus media which makes up the bulk of the total surface area available to them. (Flow rate is only a concern in a denitrator that uses a coiled air tube where the only area that the beneficial bacteria is growing on is on the inside surface of the coiled air tube where it is directly affected by the flow rate.)

What is more important than flow rate is the way the water flow. In a proper biological filteration setup, water is channelled to flow through the media evenly. One common mistake fishkeepers make is to deploy the bio media in such a way that water flows around it instead of through it. (see diagram below)

We must not forget that water flows using the easiest path it finds. Using holding bags to contain media often create gaps in between bags, allowing water to flow past the sides of the bags and little through the media. If media bags is to be used, the most efficient way to deploy them would be to have the bags closely touching all the sides of the filter chamber (see diagram below)

Biological Media (Bio media)

Other than the beneficial bacteria, the most important thing would be the biological media. Bio media is the 'land' that beneficial bacteria lives in, grow and multiply.

Anything that does not rust nor leak harmful substances into the water can be used as bio media because beneficial bacteria is not choosy. They can grow on anything that they can attach themselves to. In fact, these bacterias are on your aquarium walls, gravel, decorations, plants and even on your fish!

Due to space constraint in the filter compartment, a good bio media provides a large surface while occupying a small space. Therefore, good bio media is made of porous materials because they give a very large surface area.

If there is no space constraint, then as long as they provide enough surface area for enough beneficial bacteria to grow on, such that they keep your nitrogenous waste under control, they are good bio media!


Common bio media are:


You may be thinking, wait a minute, plastic bio balls are not porus! True, they are not porous and they do not have as much surface area as porous materials. However, if you have a large bio media compartment and do not have a tight space constraint, it is good to use bio balls because their advantage is that they are able to provide a lot of air (oxygen) for the beneficial bacteria if they are not submerged in water.

Another advantage of plastic bio balls is that they don't get clogged up because their surface areas are made up of relatively large gaps and not fine pores like ceremic material. When a ceremic material gets clogged, all that vast surface areas is gone!

Due to this reason, you should always have a good mechnical prefilter in place to provide mechanical filtration before the tank water reaches the bio filter.




Important Note on Prefiltering!

If you do not use a prefilter (mechanical filter) you can still get good results AT FIRST. This is because if you have porous bio media, they can trap dirt and debris and work like a mechanical filter. The only problem is while you can wash or throw away your prefilter(i.e. wool) easily, you cannot do the same to your bio media otherwise you will destroy the beneficial bacteria colonising on them.


Comparing Biological Medias

If you compare different bio medias OVER A PERIOD OF TIME all of them will give you the same result. The differences are

Beneficial bacteria can work better in some media because the surfaces on the media are more exposed to oxygen rich water.

Different amounts of media is required for different media because of different surface area they contain. (A more porous media with smaller holes/pores have more surfaces , but most surfaces have less access to oxygen rich water that is required for beneficial bacteria to work effeciently).

The only difference is that different media require different maintainance because some clog up easier than others.

Deployment of media is important. A media that is above the water level so that the water flows over it is more efficient. This is the more oxygen rich environment for the media.

If you compare a porous and non porous media side by side, the porous media will always show better results in the short term. This is because the micro holes and tunnels in the porous media traps tiny particles in the water. This in effect polishes the water so it looks more clear. This effect will slowly taper off as the pores of the media gets clogged up.


This is a good article about surface area.


Cleaning bio filter

Avoid cleaning all your biological filter at the same time. If there are several filter components, alternate cleaning each component. Also, it is best not to clean your biological filter under the tap due to chlorine which can kill off all beneficial bacteria. It is advisable to syphon water from your tank into a pail, and to use that water to clean your media. A good mechanical filter keeps your bio media clean and saves you the cleaning, which should be done only when absolutely necessary.


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