Particulate filtration is the collection of waterborne organic matter on or in some type of media. Polyester fiber, filter fiber, sponge, and gravel are common types of media used to collect particulates.
The best designs for collecting particulate are thin to limit the amount of surface area for bacteria to live on. When particulate filters are cleaned, much of the beneficial bacteria that have colonized the surface of the media are lost to the aquarium ecosystem. For this reason it is best if the particulate filter has a limited amount of surface area to minimize the impact on the beneficial bacteria population when they are cleaned or replaced.
Lee's Under Gravel Filter. Photo from Lee's.
Under gravel (UG) filter uses a raised filter plate that sits on the bottom of the aquarium. The plate typically has slots to allow water to pass through. Uplift tubes in the corner of the plate are either air driven or have a powerhead on top to help pull water through the plate. Gravel is used on top of the plate to act as a medium to provide a surface for beneficial bacteria to live on and provide for particulate filtration.
UG filter systems were common in the 1970’s and 1980’s, but today they are rarely used. Under gravel filtration is an adequate filtration system as long as you don't plan on keeping live plants or fish that like to dig. Many UG filters come with activated carbon or zeolite caps for the uplift tubes. I recommend that you do not use these caps, because they restrict the water flow through the gravel bed.
If you want to use chemical filtration with a UG filtration system, the best way, is to add the media to a nylon filter bag, rinse it out under tap water, flatten it out on the filter plate near the uplift tube, then cover it up with gravel.
You will need to use a gravel vacuum to pull the detritus out of the gravel bed that will build up over time. The gravel should be vacuumed at least all of the way down to the filter plate once every two weeks.