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Aquarium filters typically come in three distinct forms; the undergravel filter, the internal filter and the external filter and whilst they all do a similar job, these three types of filters are used in entirely different ways and have a range of different strengths and weaknesses.
Undergravel Filters
The undergravel filter works by placing a plastic plate on the floor of the aquarium. This plate has tiny holes in it and once placing on the bottom of the tank, it should then be covered with gravel. In one corner of the plate you will find a vertical plastic tube into which you place an aerator.
This aerator bubbles out air, oxygenating the water and the same action of the bubbles running up the tube also helps to create a gentle current which then sucks any dirt out of the water and down into the gravel where bacteria break it down.
The undergravel filter is cheap and easy ot run but requires significant setup as it can only be installed when the tank is totally empty; it is therefore not suitable for added to an existing fish tank.
In addition, the pumps can be noisy as they are housed outside of the tank and so you may not want to run yours 24 hours a day, even when in a perfect world you will. Finally, some investigation may be required to find a plate which fits perfectly into your tank as the plate should cover as much of the base as possible.
Internal Filters
Internal power filters look typically like small black boxes which are placed into the aquarium. They can be attached either using suction cups or hooks and typically the fittings are present within the packaging.
Internet filters suck water into the base of the filter, remove particulate waste and then blast the clean water back out of the top to keep the water circulating.
In my experience these internal filters have a number of notable benefits over the undergravel filter units. Firstly as they are housed within the tank itself they tend to be far quieter than undergravel filters. As they simply sit under the water at one end of your aquarium they are also easy to access and so maintain. Cleaning your filter becomes a very simple task indeed.
Internal filters are therefore especially popular and so they are low cost and easy to find. Even better they are typically labeled clearly with the size of the aquarium they are intended for so one can easily be certain of getting the right filter.
The only real downsides with internal filters are that they need to remain unblocked. This means that you should try to keep any tank décor like plants or rocks away from them and of course whilst they are normally reasonably small a few fish keepers find them unsightly and would rather try to hide them away.
External Filters
External filters are typically used for larger fish tanks where an internal filter would simply need to be too large to be practical in order to keep the water clean. Typically the water inlet and outlet are located in the aquarium and then rubber tubing carries the water to and from the main filtration cylinder.
This means that the main canister needs to find a home and can be quite large.
Due to the power of these external filters they are typically the most expensive option though arguably do the best job of filtration because one can easily add a variety of different filtration treatments to them.
For example as well as the purely physical action of pushing water through a sponge to clean it, many of these filters have areas where one can place carbon granules for example, which will further help to remove impurities and toxins from the water.
In Closing
In my years of keeping fish and experimenting with different filtration methods, my own preference is for an internal power filter. They are reasonably priced, come in a range of sizes, are easy to care for and do a good job.
To me, undergravel filters are simply too impractical and while I have used external power filters from time to time, the cost (and the business of having to hide the canister) can be too much for many people.
