The mechanical filter is just one part of  the overall waste management architecture of the aquarium.  Other parts include the biome and the periodic water exchange with its’ associated gravel vacuuming.  Making an informed choice regarding which mechanical filter is appropriate for your aquarium has a great deal to do with who your aquarium occupants will be, the volume of water that needs to be processed, and the anticipated particulate load.

If you are just beginning your research to set-up an aquarium be sure to read the guide on setting up a new aquarium for a comprehensive discussion of the many considerations a new aquarist must consider.

Of the three types of filters, mechanical, biological, chemical, the most common filter purchased is the mechanical filter.  This has much to do with availability, familiarity, and ease of use.  When people think of a filter they think of a mechanical filter sitting on an aquarium rim and spilling filtered water into aquarium.

Not too much thought is put into understanding the role of the filter or what expectations are reasonable for this filter design.  It’s only after the filter has been in use for awhile, or the aquarist becomes better informed, that decisions are made to upgrade the filtration equipment to better suit the aquarium.

This guide will walk the aquarist through mechanical filter decision points to better match the filter to the aquarium needs.

The goal of any mechanical filter is to trap suspended particulate matter in filtration media and return to the aquarium a clean water discharge.  To achieve this goal a mechanical filter relies upon three components.  They are turnover rate, filter media, and power.

Turnover rate: The rate at which an aquarium’s water should be filtered.  The turnover rate can vary depending on several factors, including the tank size, the number and size of the fish, the types of filtration media used, and the specific needs of the aquatic inhabitants. Generally, it is recommended to aim for a turnover rate of 3 to 5 times per hour for freshwater aquariums and 5 to 10 times per hour for marine (saltwater) aquariums.

To calculate the turnover rate, use the following formula:

Turnover Rate = Volume of Water (in gallons) ÷ Filter Flow Rate (in gallons per hour)

For example, if you have a 20-gallon freshwater aquarium with a filter that has a flow rate of 100 gallons per hour:

Turnover Rate = 20 gallons ÷ 100 gallons per hour = 0.2

In this case, the filter is providing a turnover rate of 0.2 times per hour, which is quite low. For a healthy freshwater aquarium, it is advisable to have a turnover rate closer to 3 to 5 times per hour. So, for a 20-gallon tank, you would want a filter with a flow rate of at least 60 to 100 gallons per hour.

For marine aquariums, especially those with more sensitive or demanding marine life, a higher turnover rate is often recommended to maintain water quality and ensure adequate oxygenation.

It’s important to consider the specific needs of your aquarium’s inhabitants when choosing the appropriate filtration and turnover rate. Additionally, factors such as the presence of live plants, the bioload (amount of waste produced by occupants), and the type of filtration media used should be taken into account when determining the ideal turnover rate for your aquarium.

Bioload: Determining the bioload is more of an art than a science.  Once the bioload is determined there is no established method to translate the bioload into sizing of a mechanical filter.  A useful rule of thumb is to use the length of each aquarium occupant and calculate a total length.  The larger the number the larger is your bioload, the smaller the less is your bioload.

The value of bioload is to provide a measure of biological waste and waste byproducts.  It’s helpful to equate bioload to the amount of suspended particulates in the water.  For example: a 55 gallon aquarium with 10 fish each 1.5 inches in length, 15 inches combined, would give a bioload of 3.6 [gallons / combined inches = bioload].  This number is a calculated value that is directly proportional to the number of aquarium occupants who are producing waste products.

When a fish is added to the aquarium, the bioload increases as does the waste products from the fish.  Bioload is helpful for the aquarist to conceptualize the particulate waste products generated from aquarium occupants.

Bioload is also expressed in terms of chemical compound concentrations and dissolved gases.

Ammonia Production Rate: One of the key components of bioload is the production of ammonia, which is a toxic waste product excreted by fish and other organisms. Bioload can be expressed as the rate of ammonia production, usually measured in milligrams of ammonia per day or per week.

iological Oxygen Demand (BOD): BOD is a measure of the amount of dissolved oxygen that bacteria and other microorganisms require to break down organic matter in the water. It indirectly reflects the bioload because more waste in the water leads to higher BOD.

Nitrate Accumulation: Nitrate is a byproduct of the nitrogen cycle in the aquarium, and its concentration can be used as an indicator of bioload. Higher nitrate levels suggest a heavier bioload, as it is the end product of the conversion of ammonia and nitrite by beneficial bacteria.

It’s important to note that bioload is not a single, fixed value but rather a dynamic and changing factor in an aquarium. It can fluctuate based on the number and size of fish, feeding habits, filtration efficiency, and water change frequency.

Turnover rate can be affected by several components of the mechanical filter.  They are inflow tubing, impeller, and filter media.

Inflow Tubing: The inflow tubing has a diameter that in conjunction with the impeller and magnetic motor will establish the water flow within the filter assembly.  For rim mounted/Hang On Back (HOB)  filters the impeller is spun through the movement of a magnetic interlock that is spun by the electric motor.  For external canister mechanical filters, water flow can be produced either by using an internal canister pump or by using an external vane pump that is self priming.

Both the input tubing and the impeller or vane pump have the ability to affect water flow in the filter assembly.  If either becomes clogged or impeded in any way, the turnover rate will be severely affected.  When selecting a mechanical filter consider what would be involved in clearing an obstruction from the impeller, pump, and/or input tubing.

The Impeller: The impeller used in many Hang On Back (HOB) / RIM mounted filters is constantly spinning when the filter is operating.  Interestingly, a spinning impeller does not impede the growth of algae or the deposition of other foreign matter at the base of the impeller.  It is necessary from time to time to remove and clean the impeller and the impeller seat where the impeller spins.

The impeller, wile made from durable materials, can break or splinter.  This can cause an imbalance and further degrade the turnover rate.  If a HOB filter is selected it would be wise to purchase an additional impeller to replace the one provided with the filter.

Waste saturated filter:  The filter media is intended to become saturated with biological waste products.  As the filter becomes increasingly saturated, its’ ability to pass water through the filter media becomes correspondingly less.  As a result, the turnover rate diminishes with a waste saturated filter.

When considering a mechanical filter for a 60 gallon aquarium the filter needs to be able to move 300 gallons per hour through filter media.  In commercial settings water pressure is measured at the inlet and the outlet sides of the filter.  Changes in pressure indicate saturation of the filter media.  In a non-commercial setting the aquarist must inspect the filter media and estimate the diminished water flow exiting the filter assembly.

Removing suspended biological waste and algae is the role of the filter media.  Similar to your furnace air filter that lets air pass through and is able to filter particles down to .2 um, water filter media is designed to allow water to pass while trapping particular waste of 20 um and larger.  At 20 um particles are less than half the width of human hair.

Water filter media is not intended to trap bacteria or viruses.  The average size of most bacteria falls within the range of 0.5 to 5 micrometers (µm).  Some bacteria can be even smaller, measuring around 0.02 micrometers, while others can be larger, reaching up to 10 micrometers or more.

Additionally, there are some exceptionally tiny bacteria, like Mycoplasma species, that have a unique cell structure and are as small as 0.1 micrometers (100 nanometers).

The primary reason for the limitation of particulate size is the density of water.  The density of water is significantly higher than that of breathable air. Density is a measure of mass per unit volume, and it is influenced by the molecular weight and pressure of the substance. Here’s a comparison of the densities of water and breathable air (at standard conditions):

Density of Water: At standard conditions (25°C or 77°F, and atmospheric pressure), the density of water is approximately 1 gram per cubic centimeter (g/cm³) or 1000 kilograms per cubic meter (kg/m³). Water is relatively dense compared to gases, and this is one of the reasons why aquatic creatures, like fish, have buoyancy and can float in water.

Density of Breathable Air: Air is a mixture of different gases, primarily nitrogen (about 78%), oxygen (about 21%), and traces of other gases like carbon dioxide and argon. The density of breathable air at standard conditions is about 1.2 kilograms per cubic meter (kg/m³). Compared to water, air is much less dense, which is why we can breathe it and why it has a lifting effect (e.g., allowing objects like balloons to float).

The choice of filter media and the particulate size it will trap is dependent upon the aquarium occupants and their bioload.  Aquariums that host aquatic creatures like turtles must plan for extraction of large amounts of biological waste.  Adult fish 12 inches or more in length have similar waste considerations.

Aquariums that have a number of one inch fish have less of a bioload problem, providing there aren’t too many fish in the aquarium.

Unlike HEPA air filters that have three different methods (interception, diffusion, impaction) of trapping particulate matter, water filters rely solely upon impaction as their method of trapping particulate matter.  Filter fibers are purposefully spaced so that suspended particulates will impact two or more fibers and become trapped between them.

The principal of trapping particulates is the same for all filter media.  Sponge or woven, suspended particulates will be stopped when they impact against two opposing structures in the filter media.

The velocity of water and the surface area of the filter both play a crucial role in deposition of particulate matter in the filter.

Filter manufacturers want to maintain a constant water flow through the filter while still trapping particulate matter.  To do this they have to adjust the amount of surface area, and the water velocity.

By having greater amounts of surface area the manufacturer can extend the usable life of the filter.  By keeping the water velocity the same across the entire surface of the filter, the manufacturers can increase the proportion of particulates that will be stopped by the filter media as opposed to pushing through as a result of higher water velocities.

When considering a mechanical filter look to see if the manufacturer has addressed water velocity.  Some manufacturers have a pump bypass method or an alert that the filter media is full.

Activated charcoal filters, also known as carbon filters, are widely used in aquarium filtration systems. They are effective at trapping and removing a variety of organic and inorganic compounds from the aquarium water. The porous structure of activated charcoal provides a large surface area for adsorption, allowing it to capture and hold onto substances through a process known as adsorption (not to be confused with absorption, which involves substances being taken up into the structure of the material).

These filters are effective at trapping the following in the water column:

Chlorine and Chloramines: Activated charcoal is particularly effective at removing chlorine and chloramines from tap water. These chemicals are often present in tap water and can be harmful to fish and other aquatic life.

Ammonia and Ammonium: Activated charcoal can adsorb small amounts of ammonia and ammonium, which are toxic waste products produced by fish and other aquarium inhabitants.

Phenols and Organic Compounds: It can remove various organic compounds, including phenols, which can be released into the water through decaying matter and waste.

Odors and Discoloration: Activated charcoal can also adsorb and remove odors and discoloration caused by dissolved organic substances in the water.

Medications and Chemical Contaminants: It can help remove certain medications and chemical contaminants that may be present in the water.

Activated charcoal filters are particularly useful in the initial stages of setting up a new aquarium or during water changes when tap water with chlorine or chloramines is added to the tank. However, it is essential to replace activated charcoal regularly since it becomes saturated with the adsorbed compounds and loses its effectiveness over time.

It’s important to note that while activated charcoal is beneficial for removing certain compounds, it does not provide biological or mechanical filtration.

Some filter media manufacturers have created multipurpose media that contains a mechanical filtration component and a chemical component in the form of activated charcoal.  These filters do provide mechanical filtration, but the usefulness of the activated charcoal layer is somewhat limited.

If the aquarium is in need of removing dissolved chemical compounds, consider a full activated charcoal filter and not a combination filter.  Providing more activated charcoal surface area will increase the effectiveness of the chemical elimination process.

The surface deposition of activated charcoal is typically a small area of the combination filter.  Full sized charcoal filters will accommodate some clogging due to normal operation while still maintaining surface area for the activated charcoal to continue reacting with dissolved chemicals.

As mentioned, the mechanical filter has a significant role to play in the overall waste management process in the aquarium.  The filter also has a strategic role in the aquarium of creating water currents and circulating water in and through the varied decor of the aquarium.

Circulating water ensures that all areas of the aquarium receives adequate filtration.  Water currents also assists in nutrient distribution, oxygen distribution, carbon dioxide distribution, and the prevention of stagnant water pockets.

If the mechanical filter is of the HOB style, the return spillway causes surface agitation enhancing gas exchange with the water.

When the mechanical filter fails, particularly the HOB type, the aquarium begins immediately to lose oxygen.

In a well-maintained aquarium with adequate oxygen levels, fish can typically survive for a few hours or even longer without a filter. However, the oxygen levels will gradually decline as fish continue to consume oxygen through respiration and waste production.

As oxygen levels decrease, fish will start to show signs of stress and discomfort. They may exhibit behaviors such as gasping at the water surface, rapid breathing, or staying near the water’s surface where oxygen concentrations are higher. In severe cases, fish may become lethargic, lose balance, or even show signs of distress.

So, it becomes crucial to plan ahead for power outages that will affect the aquarium.  The introduction of an uninterruptible power supply (UPS) is a good choice to keep the filter operating.  Filters draw small amounts of current which should translate into long run times of the UPS.  Check the run time of the UPS and then using the wattage of the HOB filter calculate the amount of run time the UPS will provide in the event of a power outage.

Example

If an aquarium has a HOB that uses 20 watts to operate and it is attached to a UPS rated at 1000 watts, the UPS will provide power to the HOB for 50 minutes.

Given: UPS Capacity = 1000 watts, Load = 20 watts

UPS Runtime = 1000 watts / 20 watts = UPS Runtime ≈ 50 minutes

By increasing the UPS rated watts to 1500 watts the runtime will increase to 1 hr 15 min

The UPS will extend the oxygen availability in the aquarium by adding the UPS runtime to the three hours of availability already in the aquarium water.

Alternatively, to extend the oxygen availability in the aquarium the aquarist should consider adding plants.  There are several aquatic plants that can produce oxygen and extend the oxygen availability in the aquarium.

Here is a short list of plants that can be used:

1. Anacharis (Elodea canadensis): Anacharis is a popular and fast-growing submerged aquatic plant that releases substantial amounts of oxygen. It is often used as a reliable oxygenator in aquariums.
2. Hornwort (Ceratophyllum demersum): Hornwort is a highly effective oxygen producer and is well-suited for aquariums of all sizes. Its rapid growth and dense foliage make it an excellent choice for oxygenation.
3. Waterweed (Elodea densa): Also known as the common waterweed, this plant efficiently oxygenates the water due to its rapid growth and dense green foliage.
4. Water Sprite (Ceratopteris thalictroides): Water Sprite is a floating or submerged aquatic plant that produces a good amount of oxygen while also providing cover for fish and fry.
5. Giant Hygrophila (Hygrophila corymbosa): This tall and bushy aquatic plant is a strong oxygenator and adds a lush green look to the aquarium.
6. Water Hyacinth (Eichhornia crassipes): Although primarily used in outdoor ponds due to its large size, water hyacinth is an exceptionally efficient oxygen producer.