The aquarium filter – how does it work then

Since yesterdays post we have had a few people contact us saying that it is all well and good talking about [tag-tec]aquarium filtration[/tag-tec] and different methods etc when there are a lot of visitors to this site who are thinking about starting a salt water aquarium and do not fully understand how an [tag-tec]aquarium filter[/tag-tec] actually works.

So here we go, this is the first of a couple of posts to touch upon the basics of aquarium filtration (what it is, how it works, methods etc)

The filtration system of any aquarium is absolutely essential. This filtration is the life blood of the aquarium system. It is especially more so in a marine tank as the life maintained is more sensitive to change. You must implement an effective and efficient filtration system.

Think of it this way, when you are building a flat pack piece of furniture you slowly assemble everything using as much patience and care as you can muster, you follow the instructions to the letter and at the end you have a fantastic piece of furniture which will take pride of place in your home.


If you skip steps or do not follow the instructions given then it won’t turn out exactly as planned and won’t function as it was designed to do.

This is exactly the same in a marine aquarium – if you miss steps out, rush or do not understand why then this is a sure fire plan to potential disaster.

So let’s have a look at what the aquarium filter actually is and why it is a requirement.

The filtration aspect of any aquarium ensures that if maintained correctly excellent water conditions can be achieved, the exact water conditions which are both expected and demanded by the species you are hoping to keep.

There are three different aspects to filtration which each perform different functions, these are:

Mechanical Filtration
Biological Filtration
Chemical Filtration

OK – still with me, lets have a look at those.

Mechanical Filtration

This aspect of filtration is normally defined as the removal of particulate matter from the water. This is important as it removes matter from the water before it has time to break down naturally and release polutants into the water column,

A simple piece of mechanical filtration is a sponge which water is passed through. The sponge traps things in it which can be later removed by the aquarist for cleaning.

A lot of aquarium filtration devices actually combine an aspect of mechanical filtration into their design – canister filters, for example quite often have a sponge integrated into the container for this purpose. Powerheads which can be used to feed a fluidised filter can have a sponge on the intake to perform this function.

In an aquarium filtered via natural filtration (for example [tag-tec]saltwater live rock[/tag-tec]) then mechanical filtration can be accomplished via sponges/floss on the weirs of even via hermit crabs, snails etc. Hermit cracs, snails, starfish etc are natures mechanical filtration.

With a mechanical filter it is essential that it is cleaned often, for example if you combine a mechanical filter with a biological filter normally you would place the mechanical filter before the biological filter to prevent waste from entering the biological media chamber. Over time the mechanical media will become clogged and the water flow will be impeded. This slower water volume will mean that the filtration capabilities are reduced. Unless you clean the mechanical filtration media on a regular basis you are at risk from this.

Another problem is that sometimes the output from the mechanical filter is required to provide some of the water flow in the aquarium. If the media becomes clogged and the water flow reduces, the crucial water flow in your aquarium is lowered. It is not recommended to use mechanical filters to create in-tank water flow especially in a marine reef environment where water circulation is so important.

The water flow in a tank is crucial as it ‘blows’ detritus etc from the rocks into the water column and allows it to be caught by the mechanical filtration. (In a reef environment water circulation also provides corals etc with food and removes any mucus build up etc which may occur.)

A clean mechanical filtration system is a requirement. The media has to be removed regularly and cleaned otherwise the problems as described above can become evident.

Biological Filtration

Biological filtration is very important as it is the life support system. Without an efficiently functioning biological system the livestock would quickly become sick and potentially die. The process of biological filtration is known as the nitrogen cycle.

The reason it is called biological filtration is because it involves bacteria that feed on the wastes created. Probably the most well known wastes are ammonia and nitrite, both of which are toxic.

One set of bacteria breaks down ammonia when in the presence of oxygen. The result of this is nitrite which is dealt with by other bacteria again in the presence of oxygen, this time the product is nitrate. This is where the cycle ceases with several types of biological filtration. (Where the nitrogen cycle continues, the nitrate is converted into nitrogen gas, but this time in the absence, or nearly so, of oxygen.)

For successful biological filtration to occur you need to provide the following:

  1. Give the bacteria a suitable place to live
  2. Give them something to eat
  3. Provide oxygen

Now that seems straightforward doesn’t it. Actually it is!

At present there is no ‘absolutely perfect’ biological media on which to base your biological filtration. However, saltwater live rock does come very close. This is why systems based upon live rock are so popular at this moment in time. The filtration capabilities of live rock are excellent. Also, as a bonus, a tank with sufficient live rock will enable the full nitrogen cycle.

All biological media must be kept free from clogging. The majority of ‘off the shelf’ man-made filtration products cater for this, as an area of mechanical filtration is included. When using live rock as biological filtration the water flow in the tank is used to keep the system clean (as well as a clean up crew and the occasional manual clean).

The most likely neglected need of biological filtration is the supply of oxygen. It is essential that the tank water has as much oxygen content as possible. This will satisfy the requirements of the biological filtration, and will be beneficial to the other organisms, big and small, in the tank.

There are some filters which provide this oxygen by having the filter media “dry”. What this means is that the media is not submersed, but the water is passed through it before returning to the tank. A good example of this type of filtration is the trickle filter.

The other end of the scale is where the filtration media is entirely submerged in water, i.e. in canister based filtration, live rock, deep sand beds etc. The only oxygen available is that which is dissolved in the tank water.

A short note on the full nitrogen cycle. As already discussed, ammonia is converted to nitrite, which in turn is converted to nitrate. If biological filtration such as canister filters are being used, the nitrogen cycle does not proceed beyond the production of nitrate. Sufficient quantities of live rock will process nitrate as will a deep sand bed.

An aquarist using for example canister filters, can control nitrate with water changes. In addition, there are special filters available to process nitrate.

Chemical Filtration

Chemical filtration is an aspect of the aquarium filter where dissolved substances are removed from the water.

The most commonly used types are activated carbon and adsorbing resins. Adsorbing resins are very often used to remove phosphate.

Activated carbon traps particles from the water mechanically, though this is not its prime purpose. The prime purpose is to chemically bond some pollutants. These pollutants are dissolved organic compounds (DOC’s). These include proteins, fats, organic acids and other defensive/offensive chemicals that are sometimes secreted by marine life.

There is an issue with using activated carbon, this is that some good trace elements are removed. Regular water changes and the addition of supplements if necessary will replace these. The adsorption capability of activated carbon is limited, therefore it will need to be replaced at regular intervals, I would recommend at least every three weeks.

A good way to use activated carbon and/or phosphate removal media is to fluidise them. This increases the contact time with the water and improves efficiency. I have made my own fluidised devices in the past which I will post in the future about.

The decision as to which type of aquarium filter to use is a personal choice. Personally I use live rock in my aquariums, mixed with other nutrient export devices. My father uses canister filters as well as external nitrate/phosphate removal devices. Two completely different types of filtration, however both aquarium systems are deemed by us to be successful.

So that’s how filtration works in a nutshell – it may be worthwhile to understand how the nitrogen cycle works and the various different types of filtration devices which can be used.

I hope that this has been of some use and of interest to you.

  1. i hav been running a marine tank for about 5 years now and i found the the info easy to read and accurate. Quite interesting well done

  2. Thanks Brett. Even better coming from an aquarist with experience.

  3. Selecting the appropriate filter for aquarium is very important as not just any aquarium filter will do. There are at least 150 different fish tank filters present in the market and all these filters are offered by the most trusted and top selling brands.

  4. I want fish that are pretty

  5. Hi Robert.

    Yes, the great attraction of tropical marine fish are their beautiful colours and also their varied shapes.

    Very important though is to ensure the fish are compatible with each other and the reef (if in a reef tank!).

    Johns last blog post..The Finger Leather Coral

Comments are closed.