How to use Undergravel filter?

Under Gravel or Undergravel Filters

Filtering Your Aquarium Water

What Is an Under Gravel Filter?

Under gravel filters are any filters where the water is drawn through the gravel as the primary filter media. The portion of the filter you will purchase is a plastic grate that keeps the gravel off of the bottom of the tank, lift tubes to deliver water, and power heads or an air pump to move the water.

How Does an Under Gravel Filter Work?

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.

What Maintenance Does an Under Gravel Filter Require?

With an under gravel filter you neither have to nor want to change your filter media. However, you will need to clean the gravel regularly. The easiest way to do this is by using a gravel vacuum and cleaning 1/4-1/3 of the gravel each time you do your weekly water change. This is necessary to prevent the gravel from "channeling," where the debris collecting in the gravel blocks some passages and forces the water through channels, resulting in decreased water flow and reduced exposure to the bacteria that clean the water. Under gravel filters are generally the lowest maintenance filters and are the cheapest to keep running.

What Should I Be Aware Of with an Under Gravel Filter?

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, 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.
Though some people claim that under gravel filters are unreliable or even dangerous, these claims are unfounded, unsupported, and irrational.