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Discussion of Basic Filter Options for the Aquarium

There are three types of filtering that must be considered for the freshwater aquarium, mechanical, chemical, and biological.  All three can be handled by one filter to some degree, however most aquarists use a combination of filters to assure all three requirements are met.  Let's review each requirement first:

Mechanical
This is the most familiar type of filtering.  It keeps the water clean and free from floating particles and debris.  Water is passed through some kind of filtering media such as a sponge, cotton or some synthetic fiber material where the debris is trapped.  The filtering media is either replaced or cleaned periodically.

Chemical
This type of filtering keeps the water free of certain toxic elements and dissolved organic compounds (DOCs).  It also helps remove odors.  Water is passed through activated carbon particles.  The activated carbon must be replaced periodically.

Biological
Probably the least understood and most important part of having a complete filtering system.  Biological filtering removes toxic ammonia created from fish waste, decaying plants, etc. and converts it to non-toxic nitrate by passing the aquarium water over a colony of nitrifying bacteria.  In the case of UGF filters, discussed below, this bacteria colony resides in the gravel or substrate.  Many power filters and most cannister filters use a bio-media within the filter to culture the bacteria. The nitrifying bacteria will grow anywhere oxygen rich water comes in contact with a surface area.  The process of culturing the bacteria and and preparing the aquarium for fish is called cycling the aquarium and is done by completing the Nitrogen Cycle.  You can read about the Nitrogen Cycle in more depth by clicking on the fish.  Not properly cycling an aquarium is the number one fish killer for the beginning aquarist.

With an understanding of the three different types of filtering required, let's look at the different types of filters available.

Internal Corner filter
A very basic filter used in smaller tanks(10 gallons or less).  This filter consists of a plastic box that sits inside the aquarium on the substrate, usually in one of the corners.  The box contains a filtering media (usually cotton) and activated carbon.  The box contains perferations for water entry and a vertical tube where the water is discharged.  An airline with an attached air stone is inserted in the outlet tube.   As the air is released from the air stone and rises to the surface, a water flow is created.  Water is drawn into the box through the perforations, passing through the filter media and the carbon then out the outlet tube.  For cleaning or media replacement, the entire filter must be removed from the aquarium.   These filters do not have an area for biological bacteria to reside.

HOT Power Filters
These are by far the most popular filters for aquariums between 10 and 75 gallons.  They come in all shapes, sizes and flow capacities.  Most have optional attachments to perform additional functions.  HOT means "Hang on the Tank".  As the name implies they are mounted on the outside back upper lip of the aquarium.  Most operate in a similar manner.  A hard plastic intake tube is place in the aquarium with the inlet close to the substrate. The top of the tube is "U" shaped and hangs over the lip of the aquarium with the outlet in the filter box.   The filter box has compartments to place the filter media, activated carbon and in some cases the bio-media.  Water enters the intake tube and flows into the filter container where a pump located on the filter outlet draws the water through the filtering media.  The water is then returned to the aquarium.  These filters are not difficult to clean or replace media.  Clean filtered water returning to the aquarium provides good surface movement promoting good oxygen exchange.  Some of these filters can be equipped with bio-wheels where part of the pump outlet is diverted to the bio-wheel to increase the surface area for bacterial growth.  This type of filter is an excellent choice for an economical filter that does a good job of mechanical and chemical filtering.   With the use of the bio=wheel or in combination with a UGF filter, they make an excellent system for the aquarium.

Cannister Filters
These are excellent aquarium filters and come the closest to provide all three types of filtering in one unit.  Rather than being placed inside the aquarium or hung on the back, these filters can be placed on the floor beside the aquarium or in a cabinet beneath it.  They consist of a large cannister with inlet and outlet connections.  The inlet tube similar to the power filter consist of a hard plastic tube attached to the aquarium wall via suction cups with the inlet placed close to the substrate. A hose is connected to the opposite end of the rigid tube and is routed to the inlet of the filter housing.  A second hose runs from the outlet side of the filter back to the aquarium for returning the filtered water.  The filter housing contains three to four compartments used for holding the filter media. bio-balls for bacteria growth and activated carbon for the chemical filtering. Other media such at peat can also be added.  Cannister filters allow you to use a variety of different media.  Most come with in-line valves or quick disconnect features permitting the filter to be disconnected for cleaning and/or filter media replacement without losing the syphon.  Cannister filters are the best choice, in my opinion, if you are looking for one filter that can do it all.

UGF Filters
UGF stands for Under Gravel Filters.   Really a specialty filter, it is designed only for biological filtering.  A separate filter must be used to achieve the mechanical and chemical filtering.

The UGF consists of a plate or series of plates that are grooved or filled with holes and placed along the bottom of the tank covering the surface completely.  One or two, uplift tubes, usually placed against the back wall of the aquarium, are connected to the plates.  Electrical power heads or air stones are placed in the uplift tubes.  Gravel to is placed on top of the plates to a height of one or two inches.  Water is drawn down through the gravel and through the plates toward the uplift tubes, where it is then dispersed back into the upper part of the aquarium by the power heads or air stones.  The oxygen rich water passing through the gravel provides an excellent environment for the growth of nitrifying bacteria, essential in converting toxic ammonia to non-toxic nitrate.

This filter in combination with the HOT power filters mentioned earlier, will provide an excellent system for the aquarium.