Under gravel filters consist of a plastic grate or "filter plate" which lies under the gravel of the tank.
This plate allows water to flow freely under the gravel.
Water is drawn through the gravel (which acts as a mechanical and biological filter).
To move the water, you can use either power heads or air pumps. A power head will pump water out of the top of the lift tube and into the tank,
and air pump will blow bubbles at the bottom of the lift tubes and the bubbles will lift water up the lift tube and into the tank.
As water is moved out of the lift tube, it is replaced with water from under the filter plate, which in turn pulls water through the gravel where it is cleaned.
The gravel it self provides mechanical filtration by catching large free-floating particles.
The gravel, as well as the filter plate, tank bottom, and lift tubes, provide a bed for the bacteria of a biological filter. Under gravel filters primarily provide biological filtration.
Some under gravel filters come with carbon cartridges that fit the top of the lift tubes.
These are unnecessary and can be dangerous. There is not enough carbon in these cartridges to provide sufficient chemical filtration,
if you are running your filter with an air pump, you are not moving enough water through the carbon to provide good chemical filtration,
and the carbon will restrict (and may even stop) water flow through the filter. Additionally, when the carbon becomes saturated,
it is possible for it to begin to release other toxins into your tank. If you do decide to use carbon cartridges with your under gravel filter,
you should replace them at least three times a month.
Remember to remove any carbon from the tank or filter before you medicate (if the carbon is working, it should remove the medication from the tank in under an hour,
in which case your fish won't get treated).
Also remember that water will follow the path of least resistance. If there is an area of the filter plate that is exposed (possibly by digging fish),
or if you have a power head on one side and an air stone on the other, you may not have any filtration from your under gravel filter.
Under gravel filters, when properly maintained, provide efficient biological filtration and adequate mechanical filtration.
I would strongly recommend an under gravel filter for the first filter in a basic tank.
You don't have to tear up your tank to clean your filter with an under gravel filter, because to clean it,
all you have to do is siphon off the accumulated debris in the gravel when you do your regular water change.