The Weald Aquatics Blog - A guide to filtration - another blog post from Weald Aquatics | Aquarium & Pond Fish Shop in Kent


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A guide to filtration

Blog post written by Gary | 12 March 2014 | Category: Aquarium equipment

Why do I need a filter?

Filters are essential piece of equipment for keeping the aquarium water in excellent condition. Also they remove waste produced by the fish, stopping the water becoming polluted, which in turn could cause the fish to develop diseases or die.

What filter will I need?

The size and type of filter you will require is dependent on the size of the aquarium, what fish you will be keeping and your budget.

How filters work

All filters have an electric pump which moves water through the filter, passing through three stages:
  • Mechanical
  • Chemical
  • Biological

Each stage will use various media to maintain water quality.

Chemical filtration

Using different types of media, either activated carbon or a resin, chemical impurities in the water can be removed effectively. Both media are either supplied loose or in net bags ready for use.

Activated carbon works by adsorption (i.e. chemical molecules attach to the outside of the carbon). This means most carbon media become filled up within a couple of months and will need to be changed to continue removing chemical impurities from the water.

Resin media are used for specific removal of chemicals in aquarium water such as phosphates and nitrates from freshwater aquariums.

There are some resins which last up to 6 months and will remove heavy metals, copper, phosphates and nitrates all in one go.

Mechanical filtration

There are a number of different types of media which trap solid waste from the aquarium water; these are either a ceramic media or sponges which are graded coarse, medium and fine.

A coarse media usually removes the larger particles while a fine or filter floss will remove very fine particles 'polishing' the water and keeping it crystal clear.

Biological filtration

This section of the filter breaks down the metabolic waste products from the fish, ensuring the water is healthy for the fish to live in. This is achieved by the filter media becoming home to friendly aerobic bacteria (Nitrosomanas), which break down the toxic ammonia excreted from the fish into the slightly less toxic nitrite, which is then broken down by another oxygen-loving bacteria (Nitrobacter) into less harmful nitrates. This action is part of a process known as the nitrogen cycle.

The media used in the biological part of the filter usually have a high bulk density allowing optimum colonisation by friendly bacteria for fast decomposition of ammonia and nitrite. However all sponges used in any filter system will be capable of housing friendly bacteria to break down the fish waste.

Types of filters

There are three main types of filtration for the aquarium:
  • Internal
  • External
  • Undergravel

Each type of filter will have some advantages and disadvantages. Choosing the correct filter is essential for keeping your aquarium in the best condition.

Internal filters sit inside the aquarium and are ideal for small to medium aquariums and are easy to maintain and inexpensive to buy.

The downside is that the filter, being inside the aquarium, takes up space and can also look obtrusive. Furthermore, their size can be restrictive when used with large messy fish such as goldfish and cichlids.

External filters are usually sited under the aquarium inside the cabinet.

As external filters are larger than internal filters they have more media inside which results in less frequent cleaning than internal filters and are more versatile with various specialist media available to improve the water quality.

External filters are usually more expensive than internal filters but will be worth paying the extra money for versatility and greater filtering capacity.

Undergravel filters take more time to keep clean and can cause problems with a planted aquarium. They are also not as versatile as an internal or external filter. However they have a larger surface area than internal filters and are ideal for messy fish and are also cheaper than internal and external filters.

Internal filters

The filter is usually fitted to the back of the aquarium just below the waterline for the best results.

Internal filters usually consist of a number of sponges with added biological media in the larger versions to cope with the added biological load which comes with a larger aquarium.

The operation of the filter is simple; water is drawn into the bottom of the filter through the various sponges which trap debris from the aquarium water. At the same time, friendly bacteria will colonise these sponges breaking down any toxic waste in the water.

Water is then pumped out into the aquarium via a variable nozzle from which the flow of water can be adjusted and directed; at this point air can be injected into the flow of water by means of a venturi system.

External filters

External filters usually fit into the cabinet under the aquarium connected by two tubes. Water is removed from one end of the aquarium, cleaned and then returned back to the other end of the aquarium via a spray bar or other spray nozzles.

Cleaning the water in an external filter is achieved through various sections. The first is the coarse mechanical cleaning which removes the larger particles, the second is biological which cleans the water of toxins and the third is the polishing which removes the very fine particles. Finally there is the chemical filtration, which takes out all chemical impurities from the water.

As the external filter has a large filtering capacity, the various sections of the filter can be adjusted to suit the tank conditions, for example, a larger coarse section for large messy fish like goldfish or specialist media to greatly improve your water condition.

Many of the new external filters now have a self-priming mechanism which allows water to be drawn from the aquarium into the filter ready to be pumped back into the aquarium. This device makes life easier when cleaning or servicing the filter.

Undergravel filters

An undergravel filter is normally a plastic tray that has a space underneath with gravel on top which is fitted to the bottom of the aquarium. One or more uplift tubes are fitted to the plastic tray through which water is drawn, either by a water pump (known as a powerhead) or an airstone powered by an airpump. The action of the water being drawn through the uplift tube pulls the water through the gravel which removes particles and toxins keeping the water clean.

Servicing filters

Most internal filter sponges will need to be cleaned every two to four weeks depending on stock levels and the type of fish kept in the aquarium. When cleaning the sponges from an internal filter, they should be rinsed in water that has been taken from the aquarium. Doing this will retain the friendly bacteria in the sponge. If the sponges are washed in tap water the chlorine in the water will kill off the friendly bacteria and this will lead to a rise in the potentially toxic ammonia and nitrite, which in turn could cause the fish to develop diseases or die.
External filters will probably need less cleaning as they have a larger volume than internal filters, so will be able to collect more debris over a longer period.

Impellers which are fitted to all pumps both external and internal will need to be cleaned regularly. Refer to the manufacturer's instructions on how to do this.

For external filters, the feed and return pipes will need to be cleaned. This can be done with various sized flexible pipe brushes.

Maturing a filter

When a new filter is added to an aquarium, as previously pointed out the friendly bacteria have not colonised the filter.

If the filter is to replace an already working filter, the best solution is to run both filters together for about two weeks before removing the old filter, as this will allow the new filter to mature quicker and also keep the water in excellent condition.

If the filter is in a new tank without fish, it can be matured either by using a maturing liquid or using hardy fish such as Zebra Danios.