Corner Filters / Box Filters
These filters are usually sold with cheap, small setups and usually fail to support an aquarium. The design is a small box filled with carbon and filter floss. Water is pulled through the box and back into the water. These units sit inside the aquarium and are powered by an air pump. They are designed for mechanical and chemical filtration, and support a small amount of bacteria, until the floss and carbon are changed. I have had good results by removing the floss and carbon, replacing it with gravel or other biomedia. This will allow for a limited amount of bacteria growth, capable of supporting a small fish load. These filters work well for raising fry. You are best to simply purchase a sponge filter which is about the same price, yet much more effective.

Sponge Filters
These are also an internal form of biological filtration. Water is pulled through a sponge and then pumped back into the water column. As the water flows through the sponge bacteria grows, as with the UGF. Sponges are very inexpensive and capable of supporting a very large bioload, provided your fish are not extremely large. Sponges tend to become covered with waste and normally need to be cleaned on a monthly basis. Cleaning the sponge only takes about 30 seconds. When doing your regular partial water change, you can remove the sponge and rinse it out in the bucket of dirty aquarium water. This will allow you to remove any debris from the sponge without harming the bacteria bed. Sponge filters are excellent for aquariums where a gentle water flow is required. They are also excellent as a secondary biological filter, which can easily be placed in a hospital tank for immediate biological filtration, should the need arise. I currently run 35 aquariums using the Sponge filter as the only form of filtration. I find them to be very easy to maintain.

Trickle (Wet-Dry) Filters
A trickle filter sits underneath the aquarium stand. Water is gravity fed to the trickle filter where a bacteria bed is supported. This bacteria can consist of bioballs, a biowheel, or other biological media. The key is that all of the media should be held out of the water. Water is allowed to "trickle" over the biomedia while being exposed to air. The result is an extremely large amount of bacteria growth, capable of supporting extremely large fish loads. Most trickle filters will have a prefilter which serves for mechanical filtration. If you decide to design your own model, it is very important that all of the biomedia be out of the water. Submersed biomedia defeats the purpose of a trickle filter and you may as well use a sponge filter.

Power Filters
These filters hang on the back of your aquarium. Water is pulled into the filter and pushed through a filter pad containing carbon. The result is mechanical and chemical filtration. These are usually not a very good source of biological filtration. However, many designs have incorporated a biological filtration pad within the unit, and thus become a complete filtration system. These include the Whisper, AquaClear, and Millennium units. I have personally had mixed results using these units for biological filtration purposes. The only power filter unit I have used which has an excellent biological filter are the Penguin models, which are discussed below. The power filters are primarily meant to accent current biological filtration, such as a sponge filter or undergravel filter.

BioWheel Filters
These products are primarily produced by Penguin and combine a biological filter with the advantages of mechanical and chemical filtration. They consist of a unit which hangs on the back of the aquarium. Water is pulled into the unit by an impeller and then discharged back into the aquarium. The biowheel itself is the last step in the filtration process. It grows enormous amounts of bacteria and is capable of supporting a large fish load. This is my preference for aquariums which house large cichlids. This filter line also includes a mechanical filter pad. This pad is the first step in the filtration process. As water flows through the pad large particles are removed, keeping the biowheel free of debris. The filter pad needs to be rinsed off or changed on a regular basis. Also, inside the filter pad, is a layer of activated carbon. The carbon removes odors and other impurities from water. This will keep your tank from smelling fishy, as well as keep the water crystal clear. I am very partial to the Penguin line of filters, especially the Emperor models. These provide the best of both worlds all in one unit: biological and mechanical / chemical filtration.

Canister Filters
These filters are rather expensive but usually of very good quality and efficiency. Water is taken from the aquarium through a hose and into a tight-locked container which can rest anywhere outside the tank. In the container it passes through several compartments which allow you to configure the filtration media any way you want. A water pump then pushes the water back into the tank through another hose. This type of filter is very versatile and allows you to enhance biological, mechanical, or chemical filtration according to your tanks necessities, or keep a good balance of all three in a single unit.

Fluidized Bed Filters
These filters are growing in popularity and deservingly so. They consist of a cylindrical pipe filled with sand. Water is pumped through the filter and the sand, resulting in massive bacteria growth. They are an amazing source of biological filtration contained in a very small unit. Aquariums up to several thousand liters can be filtered with these units. They do require a prefilter to remove particulate matter. The prefilter should be cleaned weekly to allow for the filter to maintain its water flow rate. These units are best used in conjunction with a power or mechanical filter of a high flow rate.