In my early years of fishkeeping, I started from basic plastic jars with only sand, water, and fish. When I started learning about filtration, my fascination with it never faded; I’ve even designed my own filtration system. But back then, undergravel gravel and a corner box filter were the only two filters available. Hobbyists today are so fortunate as numerous filter designs are now available.

Choosing a filtration system for an aquarium is a major decision. It will affect not only the type and quantity of fish, it will also affect the amount of time needed to maintain the tank.

There are three types of filtration that are crucial for a well maintained aquarium. Most filter designs focus more on the first two systems.

Mechanical Filtration

This is the process through which solid waste is removed from water. To achieve this, aquarium water is forced through a medium that is designed to hold these elements. Today, there are many natural and synthetic media available. This filter media is available in organic form, which includes gravel, ceramic materials, pumice, and lava stones. Commercial filter media that has been developed and become popular include foam, filter floss plastic, bio balls, and cylinder shaped types called “K1 kaldnes.”

Biological Filtration

This involves different types of bacteria converting toxic waste into a by-product with less toxic yield. This process is called the nitrogen cycle (Editor’s note: There is an extensive discussion of this in Angel L. Ampil’s column in this issue!).

Chemical Filtration

This removes toxic or unwanted chemicals as the water passes through a chemical media. Common media used are charcoal and zeolites.

Types of Filters

Under Gravel Filters

How It Works: There is a mechanical filtration effect as the water flows through the substrate, which traps debris. Biological filtration is obtained when bacteria strives living on the surface area of the substrate that the water is passing through. This, however, is limited due to the lower oxygen content of the water passing through the substrate. The disadvantage of the under gravel filter is the uneven flow through the substrate due to media depths and decor. Dead spots cause anaerobic bacteria, which is unhealthy for fish. (Editor’s note: Fellow columnist Angel L. Ampil discussed these deadly anaerobic spots at length in our December 2017 issue.)

Features: These are normally made from a plastic slotted plate that is installed underneath a substrate such as gravel and sand. It has an advantage in that since it is covered with gravel, it is mostly out sight, making for a better-looking aquarium. Uplift tubes are extended upward toward the surface of the water. Siphon hoses are connected directly to the tube; others, with an air stone driven by an air pump. Power heads are also used, placed on top of the tube which, when correctly placed in the upper end of the pipe of the under gravel filter, creates a bigger water flow, thus resulting in more effective filtration.

Maintenance: After a while, the slotted plates will be full of debris, and will require a general cleaning of the substrate. The substrate should be regularly maintained as it is easily clogged. When your filtration material becomes dirty, clean the substrate using gravel cleaner. By siphoning the gravel, all the debris and organic waste build up goes down the drain into a bucket. Once the gravel is clean, your filtration material is ready to be used again.

Pros and Cons: They are not appropriate for use in reef tanks. Ideal substrate thickness is around 5 centimeters or cm in height. Also, note that the under gravel filter does not provide chemical filtration, another stage of filtration process.

The use of an air pump is more effective on smaller tanks as opposed to large ones. Be sure to use an air pump that has sufficient volume to draw the water through the media and an under gravel plate.

The under gravel filter is not as effective as other filtration systems when used as a stand-alone filter. It might be useful enough for smaller-sized fish tanks, but for bigger tanks, it may be used as an additional filtration. I suggest using Hang On filter, canister, and overhead as support for under gravel filtration.

Sponge Filters

Features: As the name implies, these are sponges through which aquarium water is drawn. These filters have a replaceable sponge and come in a variety of shapes and sizes. Most popular is the cylindrical shape. The sponge density and pores vary as well. They can be powered by air pumps, or power heads.

HOW IT WORKS: The sponge material is capable of both mechanical and biological filtration. Mechanical filtration occurs as the sponge traps particulate matter suspended in the water that is passing through it. The beneficial bacteria living on the sponge provide biological filtration. Its mechanical and biological effectiveness is limited, though, due to the lower amount of oxygen and volume of water drawn through the filter.

Maintenance: This type of filtration is very inexpensive and easy to maintain. Every week the sponge filter should be rinsed in water removed from the aquarium. Using old aquarium water is important when cleaning these filters so you do not risk killing the beneficial bacteria that are responsible for the biological filtration.

Pros and Cons: They are effective in small aquariums with a small number of fish. They are ideal for bare tanks, which is why fish distributors and wholesalers use them often. (Bare tanks allow them to quickly catch large numbers of fish, which is important to them for efficiency.)

Sponge filters also work well in tanks used for :

  • Breeding – Breeders prefer sponge filters in their fry or nursery tanks as these prevent the fry from being sucked up or breeder fish from being injured.
  • Fry grow out – They provide safe and gentle filtration where it is needed, such as in a fry tank where young fish could be sucked into the intake of standard filters.
  • Shrimp and crayfish set ups – May cherry red shrimps love my big sponge filters as they are safe from being sucked in; they also devour the algae and uneaten food that adheres to it.
  • Start up – Once a new aquarium is set up, a mature sponge can be placed in a bag of water and transferred directly to a new tank, thus maintaining the good bacteria.
  • Slow moving fish – Fish species such as discus, angelfish, and even bettas that do not thrive in strong currents benefit from sponge filters.
  • Hospital or quarantine – Sponge filters help maintain a slow current for weak and stressed-out fish.
  • Pre filter for canister, overhead filters – Sponge filters work well as a pre-filter on the inlet of a canister filter. The sponge filters out a good deal of the larger particulate matter, which keeps the canister from clogging. It is far easier to clean or replace the sponge pre-filter often, rather than tearing apart the canister filter. Additional biological filtration is also provided this way, and the sponge is ready for use in setting up an emergency aquarium should the need arise.
  • Single fish show competitions – Easy set up and ready colonized bacteria are easily transferred

Maintenance: Always check the connection between the flexible hose and the filter box because they are prone to loosen up and might disconnect and stop the filtration flow.

Pros and Cons: The flexible hose is not a good sight in any tank. I had a bad experience with the external pump because it overheats and my fish room almost caught fire. Fortunately, my smoke detector worked. Submersible filters are much safer to use.

There are two benefits to using overhead filters. First, beneficial bacteria are able to work very effectively due to the excellent air and water interaction via the filter media. Second, maintenance is a breeze. The cover lid can be opened and the filter media can be cleaned or replaced without turning off the power.

Hang On Back Filters (HOB)


Features: These filters literally hang on the back of your aquarium and suck tank water up through an intake tube.

How it Works: The water passes through several chambers, depending on the design for the filtration process. After being filtered, the water is then returned to your tank through some kind of waterfall spout.

Maintenance: The maintenance required by hang-on filters is easy, and the same as with an overhead filter.

Pros and Cons: Hang On Back filters here are mostly used for planted and small community tanks. A combination of HOB and under gravel filters are my suggestions for medium size tanks.

Canister Filters

Features: These are a pressurized unit that is normally placed externally under the aquarium and features the three types of filtration. The most common designs include pumps, media filters, hose, and other attachments. Models that are more expensive even have UV lights and thermometers.

Maintenance: Cleaning the canister filter, in my experience, is the main downside. It’s a challenge, from the disconnection of the tubes to the opening of the top cover. The number of brands and designs available also creates issues because you always need to have a spare gasket or rubble seal for the filter unit.

How It Works: When aquarium water enters the filter, it will first pass through a mechanical media such as floss and be forced through the chemical media. The water then enters the chambers containing the biological media where the nitrogen cycle is completed before water returns to the tank. The mechanical filtration of canister filters is superior to other filters because it can force the water through pressure to the mechanical media trapping finer debris.

Pros and Cons: Large aquariums benefit from canister filters more because of the significant volume of water they can process at a time. It can also be combined with other filter types such as the overhead and trickle filters, thus creating more efficient water filtration.


This appeared in Animal Scene magazine’s May 2018 issue.