There are many many types of filters for aquaria. I will post about a variety to help shrimp keepers make an informed decision for their aquaria. First is…
The under-gravel filter is a very old design. Panels are placed at the base of the aquarium with a funnel connected to one, and a substrate is applied on top. An air stone is placed at the bottom of the funnel and connected by tubing to an air pump. When the air pump is turned on, air bubbles up the funnel and creates a flow through the substrate and panels, and up the funnel. Alternatively, a powerhead is connected to the top of the funnel.
Mechanical filtration – This depends on how fine the substrate is. The finer the substrate, the greater the mechanical filtration. This is because with large grain substrate the particles in the water easily pass through, whereas with smaller grain the particles are more likely to be held in the substrate. This can be problematic, however. Particles can lead to clogging of the substrate, restricting further filtration. Finer substrates can also become compacted over time and also clog the filter. Make sure the substrate grains are large enough to prevent being pulled through the panel slits!
Biological filtration – The nitrifying bacteria occupies the surface of the substrate grains, so finer substrates give more efficient biological filtration as it provides a larger surface area. Under-gravel filters give a very large surface area and are efficient at this type of filtration. With fish, under-gravel filters can become clogged with excreta, but this doesn’t tend to be a problem with shrimp due to their low bioload.
Chemical filtration – Under-gravel filters typically lack this type of filtration. However, they can be adapted by putting layers of chemical filtration media in the substrate (such as zeolite and activated carbon). They can not be removed or replaced once applied.
Further notes – This filter type can not be removed or cleaned in aquaria. Some breeders connect the funnel of the under-gravel filter to the outlet pipe of external filters to create a reverse under-gravel filter system. This helps to prevent the substrate clogging.
My opinion – It is in my opinion than under-gravel filters are very useful, but should not be relied upon on their own. They are particularly useful for getting the full use out of active soils that alter water chemistry by ensuring a good flow of water throughout the substrate.