All work on the same principle where water is drawn through the lower part of the filter via means of an impeller. This lower section houses filter media usually comprised of sponges and sometimes also activated charcoal.
Once cycled through they will filter biologically, mechanically and chemically where activated carbon is also included.
Internal filters can range in audibility depending on their size, design and age They are quieter when placed completely below the water line due to the dampening affect of the aquarium water.
The larger the capacity of the filter the more likely it will produce a sound that is audible. if you are aiming for a less noisy aquarium consider choosing a pair of smaller filters rather than one higher output device.
Internal filters are easy to use and have fairly simple moving parts. Usually the impeller is the source of any trouble. Even so you should always unplug your filter before inspection.
Once warn the internal impeller can start to rattle and lose efficiency. At this time you will need to replace the impeller or the entire unit to maintain best noise level and strength.
Individual brands can differ greatly in their filtration ability, sound and durability. Well established brands may be more expensive but could be more worthwhile in the long term. Most people are satisfied with the less expensive brands as they meet their needs and budget.
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