Best Aquarium Filter Reviews and Buying Guide

best aquarium filters

Gazing into your aquarium, watching the fish gliding leisurely among the plants, you might think: “What a wonderful, natural little ecosystem I have created.”

A pleasant thought, but wrong!

What you are actually looking at is a very fragile, artificial environment that would begin to deteriorate within hours if external assistance was cut off. If you have any doubts about this, just unplug your best aquarium filter and discontinue water changes for a few weeks, and watch what happens.

Perhaps a bit melodramatic, but I’m sure you get my point.

Without an appropriate aquarium filtration system to remove harmful substances from recirculated water, or extremely large and frequent water changes, a variety of harmful pollutants builds up. In just weeks, everything in the fish tank, except for some bacteria, would be dead.

Fish do not have this problem in their natural habitat because the seas, lakes and rivers are constantly removing pollutants and debris. That’s why it’s necessary to have some way of filtering the water in an aquarium.

Choosing the best filtration system for your tank

There are many types of fish tank filters available to the aquarist such as:

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  • Internal box filters
  • External power filters
  • Canister filters
  • Undergravel filters
  • Trickle filters
  • Live rock
  • Fluidized bed filters
  • Protein skimmers

To make things even more bewildering for the would-be aquarium keeper, there are also water sterilization techniques such as ultraviolet (UV) sterilizers and ozonizers.

Which are the best aquarium filters to buy in 2020

The Importance of Good Aquarium Water Filters

As an aquarist, you are interested in seeing your fish thrive, not just survive. For aquatic animals, good health is synonymous with good water quality.

best aquarium filters

The filtration system, in turn, is the single most important factor in maintaining good water quality. I would estimate that more than 80 percent of all fish health problems in aquariums are directly linked to improper or inadequate filtration.

Although really poor water quality, such as high levels of ammonia, can kill fish quickly, many water quality problems are more subtle.

As the water quality slowly deteriorates, the fish suffer from physical stress. The stress may not be enough to cause any immediate harm, but chronic stress, over time, will cause the health of the fish to decline.

For example, a low concentration of oxygen in the water, although sufficient to keep the fish alive, will cause the fish to breathe more rapidly, changing the heart rate and blood chemistry. Growth will cease, reproductive development and behavior will decline, and ultimately, the fish will die prematurely.

Chronic stress makes fish vulnerable to bacterial and fungal infections and parasitic infestations that, under better conditions, they could normally fight off.

An appropriate filtration system and good aquarium management can minimize, if not completely eliminate, chronic stress.

To better illustrate the correlation between fish health and filtration, we will discuss the three basic types of pollutants that accumulate in aquarium water and the corresponding filter options for each kind. Particulate and organic pollutants are examined below:

1. Particulate pollutants

There are a number of sources for the particulate matter that accumulates in aquariums: uneaten fish food, digested waste products, and cell material from fish and plants. Some of this material remains in suspension rather than sinking to the bottom, making the water turbid – cloudy and dirty.

Larger particles sink to the bottom, only to rise again and cloud the water when disturbed by fish or aquarium maintenance activities.

  • Besides being aesthetically unappealing, fish and plants are adversely affected but turbid water. Turbidity-induced stress weakens the ability of fish to resist the disease organisms that are a normal part of the aquatic environment.
  • Gill filaments may become irritated and begin to swell. This interferes with respiration (resulting in additional stress), and eventually causes gill membranes to become susceptible to bacterial and parasitic invasion.
  • Deposits of particulates on plant leaves reduce light adsorption and gas exchange, and many plants will die as a result. Finally, suspended particulates may also support blooms of pathogenic (disease-causing) bacteria.

Although daily water changes of 25 percent, along with in-tank gravel cleaning, would reduce particulate pollution, few aquarists would be willing to maintain such a strict regimen for long. In addition, water changes are also stressful to some fish.

What’s the best way to deal with particulate pollutants?

The best way to deal with suspended material in the water is with mechanical filtration, which removes particulates by continuously circulating aquarium water through a mechanical screening medium.

Mechanical filtering mediums include nylon or polyester floss, foam sponges, fiber pads, aquarium sand and diatomaceous earth.

These media differ primarily in terms of the size particles they can effectively capture, cost, service life, and re-usability.

At one extreme is floss, which tends to trap only the largest particles and cannot be reused, but is inexpensive.

At the other extreme is diatomaceous earth, which can trap extremely fine particles but is expensive and requires the most maintenance. (In fact, diatomaceous earth filters are generally used for only an hour or two to clean up really dirty tanks and “polish” the water.)

The particulate-trapping ability of mechanical filtering media is determined by the size of the spaces – or pores – in the media, the shape of the spaces, and the roughness of the material. Filtering medium that traps very fine particles will quickly clog and requires frequent replacement.

Otherwise, the water flow will be restricted to the point where turbidity in the fish tank actually increases!

A better method is to use filter material that traps larger particles, such as fiber pads. Over time, the accumulation of larger particles fills in the spaces, and smaller particles are also trapped. When the water flow of the filter is significantly reduced, the filter media is then cleaned or replaced.

Unless you have very special filtering requirements, the choice of the best aquarium filters and screening media is largely one of personal taste. There are, however, a few points to consider:

  • The larger the surface area through which the water flows, the greater the particulate trapping rate will be. In addition, it will take longer for the filtering medium to clog, reducing the amount of maintenance.
  • The thicker the medium, the greater the mechanical trapping ability (all else being equal). Reusable media improves with use because you can never rinse out all of the trapped particulates. This makes the medium more efficient at trapping smaller and larger particles. For me, this suggests that wide foam blocks or fiber pads would work best.
  • An important factor in choosing the mechanical filter itself is the flow rate. An outside power filter should circulate, in gallons per hour, four to six times the volume of the quarium in gallons. Thus, for a 50-gallon tank, the filter should process at least 200 gallons per hour. When choosing from among several filters of the same flow rate, note which design offers the largest filtering-medium surface area (e.g., a filter pad area of 12 inches by 4 inches rather than 6 inches by 5 inches).

2. Organic pollutants

A variety of substances such as proteins, amino acids, phenolic compounds, pheromones (hormones that affect the behavior of other fish) and other metabolic byproducts are continuously being discarded into the aquarium water by the fish.

These organic substances are dissolved into the water – thus the name, dissolved organic carbon. If these DOCs are allowed to become concentrated in the water, the health of the fish will suffer.

For example, laboratory studies have shown that high levels of DOC are associated with reduced fish feeding rates, slower growth, decreased reproduction rates, lower immune system activity and blooms of pathogenic bacteria in the water.

It is now believed that many of the fish health problems thought to be the result of high nitrates are actually caused by DOCs. (Nitrate is an inorganic nitrogen pollutant).

Dealing with organic pollutants in fish aquariums

DOC concentrations can be controlled by 50-percent water changes every day. Few aquarists, however, are prepared to accept such a rigorous maintenance schedule. The practical way to deal with DOCs is by chemical filtration techniques that are reliable and economical; carbon filtration and foam fractionation.

Carbon filtering

Carbon filtering removes DOCs via adsorption. As the aquarium water flows through the carbon medium, the DOCs come into contact with the surfaces of the carbon granules and become attached.

(This should not be confused with absorption, where molecules are taken into the pores of the media; adsorption occurs on the surface of the carbon.)

Some DOC compounds are adsorbed directly, whereas others combine chemically with already-captured substances. When the surfaces of the carbon become saturated, it must be discarded and replaced with new carbon.

Many types of carbon filters are sold for aquarium use, but only a few are actually capable of adsorption in water.

The material of choice is high quality granular activated carbon (GAC). This is carbon that has been degassed in an oxygen oven at temperatures close to 2,000 degrees Fahrenheit. The degassing enlarges the apparent surface area of each carbon granule, greatly increasing the adsorptive capacity.

In a filter, the GAC should always be placed after the mechanical filtering medium. Otherwise, the particulates in the water will quickly coat the surfaces of the GAC, rendering it useless.

Deep beds of GAC are always more effective than shallow ones of equal area. For a given amount of GAC, the smaller the granules, the greater the adsorptive capacity.

Unfortunately, you cannot improve the adsorptive capacity of large GAC granules by breaking them into smaller pieces, because the newly exposed surfaces will not have been activated. Nor should you use powdered activated carbon. It compacts too easily and seriously restricts water flow except in specially designed systems.

There should be 4 to 10 grams of GAC for every gallon of water, and it should be replaced monthly. Choose the best quality GAC you can find (such as Marineland).

If your filter uses prepackaged cartridges of GAC, be aware that these cartridges contain only about 20 percent of the recommended amounts of GAC. The easiest remedy for this is to slit open the cartridge and fill it with more GAC.

If you have an undergravel filter and it came with little carbon-filled chambers that attach to the top of the air lift tubes, throw them away. They do not contain enough carbon to affect the DOCs and they also restrict the water flow through the undergravel filter.

Foam fractionators

Foam fractionators are usually seen in saltwater aquariums, but they work just as well in freshwater tanks. The only difference is that the freshwater aquarium must be set up and running for a while before foam fractionation will work.

GAC beds and foam fractionators will remove other harmful substances from the water, such as hydrogen sulfide, which is produced by bacterial decomposition of organic wastes in the tank. Foam fractionators have also been found to remove small particles, suspended algae and bacteria from the water.

Certain toxic chemicals that may find their way into the aquarium will be removed by GAC or foam fractionators.

At the same time, these two chemical filtration techniques will also remove beneficial chemicals, such as many fish medications. Therefore, it is imperative to remove all GAC from the filter and shut off a foam fractionator before medicating fish.

In the second part of this article, I will discuss biological filtration and the role of nitrifying bacteria in maintaining good water quality. We will also examine how to best combine the various types of filtration into a complete system.


Water changes are an important part of good aquarium management, but only in conjunction with the operation of a complete filtration system.

Regular water changes alone will not reduce aquarium pollution loads to safe levels for more than a day or two.

First, choose the fraction of the tank volume to be replaced per change - say 20 percent - and find that point along the bottom axis.

Next, decide whether changes will be made daily or weekly. Let's choose weekly, and move from the 20 percent point on the bottom axis to the point directly above on the "weekly" curve.

Now, move across to the vertical axis and read the Average Pollutant Load. In this example, it is 30 days. This means that even with 20-percent water changes every week there will always be about 30 days of accumulated pollutants in the tank. This is not an acceptable amount.

Moving to the daily curve, you can see that 50-percent water changes produce an average pollution load of 1.5 days accumulation.

Although this should be okay for removing particulates and organic wastes, it will probably not suffice for maintaining ammonia and nitrite amounts at safe levels in the average aquarium. The graph clearly demonstrates that without a filter system, even frequent, large water changes will not be enough to minimize all aquarium pollutants.

Understanding The Nitrogen Cycle

In the first half of this article, I discussed the sources of particulate and organic pollutants and how to control them with appropriate filtration.

I noted that effective mechanical filtration removes solid materials from the water, and that chemical filtration using granular activated carbon (GAC) adsorbs dissolves organic carbons (DOCs) from the water. As important as these forms of filtration are, however, they are not substitutes for biological filtration.

Biological filtration deals with the presence of ammonia and nitrite in the aquarium water. Ammonia and nitrite probably account for more health problems and deaths with aquarium fish than any other cause.

Approximately 60 percent of the ammonia in an aquarium is excreted by the fish directly through the gills. The remainder is produced by bacterial breakdown of fish feces and urine, uneaten food, dead animals and plant byproducts. Nitrite is a waste product of the bacteria that use the ammonia for food.

The accepted theory is that ammonia in aquarium water hinders oxygen uptake in the fish’s blood. Lethal concentrations cause chronic stress. Gill filaments become irritated and begin to swell. This cuts off the oxygen supply to the membrane cells, when then become infected with bacteria. Kidney damage has also been observed. Ammonia-related stress also lowers a fish’s resistance to parasite and bacteria attacks, reduces food intake and growth, inhibits reproduction, and can result in death.

Fish differ in their sensitivity to ammonia, and the toxicity of ammonia depends on the relative proportions of two forms of ammonia present in the water: ionized ammonia (ammonium ion: NH4+) and un-ionized ammonia (NH3).

NH3 is considered to be more toxic to fish than the ionized form. Some fish are so sensitive to NH3 that they are adversely affected by levels above 0.005 milligrams per liter (mg/L) of water. Because the relative amount of NH3 in comparison to NH4+ increases as pH and temperature increase, it isn’t possible to specify a “dangerous” level of total ammonia without noting the corresponding pH and water temperature.

An ammonia test kit measures total ammonia, which in a well-managed aquarium should be immeasurable.

All aquarists should have an ammonia test kit and use it on a regular basis.

Nitrite toxicity also involves blocking oxygen uptake in the blood. As nitrite levels increase, the concentration of methemoglobin (as a percentage of total hemoglobin) goes up, reducing the capacity of the blood to carry oxygen.

Nitrite levels in excess of 0.1 mg/L are considered dangerous to many ornamental fish. Sublethal concentrations produce chronic stress similar to ammonia. Here again, regular use of a nitrite test kit is an absolute requirement for a fishkeeper.

Ion exchange filtration

Although massive daily water changes and the use of chemical ammonia removers can keep the amounts of ammonia and nitrite at safe levels, both methods are impractical for long-term aquarium management. There are two basic solutions to this problem: ion exchange and biological filtration.

Ion exchange is a chemical filtration process that removes ionized ammonia (NH4+) from the water by swapping it for an ion in the exchange medium. The ion exchanger removes NH3 indirectly. Because the proportion of NH3 (unionized ammonia) to NH4+ remains constant at a given pH and temperature, removing NH4+ from the water causes NH3 to convert to NH4+.

Both natural zeolites (which look like little chips of cement) and synthetic resins (which look like little plastic beads or chips) are used to remove ammonia from aquarium water. Ion-exchange resins (but not zeolites) also remove nitrites and nitrates.

The exchange medium is placed in a compartment of an outside power filter or canister filter (after the compartment with GAC so that the water flows through the GAC first) and aquarium water is forced through it.

The amount of ion-exchange material required varies among the products on the market, so it’s best to follow the manufacturer’s instructions. (Ion-exchange media can only be used in freshwater tanks.

The presence of salt vastly diminishes the capacity to remove ammonia. This also precludes the use of salt in freshwater tanks for medicinal purposes when using ion exchange. Ammonia ion exchange is also severely inhibited in hard water.

Even moderate levels of DOC in the water significantly reduce ammonia ion exchange. Thus, the use of zeolites or ion exchange resins requires continuous filtration of particulates and DOCs.

When ion-exchange material becomes saturated with ammonia, it must be recharged or replaced. Otherwise, the ammonia concentration will quickly build up to dangerous levels (known as ammonia break-through).

Therefore, the use of ion-exchange media requires regular and frequent ammonia testing. Saturated media can be economically regenerated by a 24-hour bath in a 10-percent salt solution.

Regeneration, however, only reestablishes about 60 to 75 percent of the original capacity.

Biological filtration

An alternative approach is biological filtration, a method that uses naturally occurring nitrifying bacteria to detoxify nitrogenous wastes. One type of bacteria (_Nitrosomonas spp._) converts ammonia to nitrite. These bacteria are usually well established in aquarium gravel, on tank walls and on plant surfaces within two weeks of adding fish to a new setup.

Another naturally occurring bacteria (_Nitrobacter spp._) converts the nitrite into a far less harmful form of nitrogen waste: nitrate.

Laboratory studies have shown that nitrates are far less harmful to fish than once believed. Concentrations in excess of 400 mg/L appear safe for fish. (Nitrates are more toxic to marine invertebrates.) _Nitrobacter spp._ are usually well established in the aquarium about four to six weeks after the introduction of animals and plants.

Although all aquariums have some amount of natural biological filtration occurring on the surface of the gravel bed, it is usually far too little to handle the fish load. A dedicated biological filter is significantly more effective. There are two basic designs: the undergravel filter and the trickle filter.

Undergravel Aquarium Filters

An undergravel filter is a slotted plate under the aquarium gravel. Air bubbles or powerheads are used to pull water down through the gravel and up through the lift tubes.

The bacteria live on the surfaces of the individual gravel grains and “process” the water as it goes by, converting ammonia to nitrite and nitrite to nitrate.

An undergravel filter plate should cover the entire bottom of the tank, and the gravel bed should be 2 to 3 inches thick. Gravel should have an average diameter of 3 millimeters (1/8 inch) to ensure lots of surface area for bacteria to grow on while providing enough space among the pieces to permit good water flow with minimal clogging.

Water flow through the filter should be between 0.5 and 1.5 gallons per minute for each square foot of aquarium bottom area.

Trickle Aquarium Filters

Trickle filters (also known as wet/dry filters) work on the same principle as undergravel filters, but implement the concept differently.

The trickle filter’s biological bed is outside of the tank. Aquarium water enters at the top of the filter column and trickles downward through the medium (e.g., stone, specially designed plastic media, fiber materials, etc.), which is not submerged.

The nitrifying bacteria inhabit the surface of the medium and process the water as it trickles by. The water then collects at the bottom of the filter column and is pumped back to the tank.

This design provides for efficient nitrification, as well as good aeration of the water. Some of the better commercial trickle filter designs incorporate drawers for mechanical filtering media and GAC, so that the entire filtration system is built into one unit.

Trickle filters are particularly well suited for marine aquariums, where dissolved oxygen levels are low, and freshwater aquariums that use a soil substrate for plantings, making undergravel filtration impossible.

Trickle filters do not, however, possess inherently higher nitrifying capacities than undergravel filters. When the most relevant variables are matched, trickle filters and undergravel filters are equally effective.

For best operation, the volume of the trickle column should equal about 8 to 10 percent of the tank volume. The water flow rate should be several tank volumes per hour. For the most efficient nitrification, the filter surface area should be designed to yield 0.5 to 1.5 gallons per minute per square foot.

Downsides of biological aquarium filters

The drawbacks of biological filtration are mostly associated with startup – what is popularly known as “new tank syndrome.” There is a lag between the time the fish are placed in the tank and the time the populations of bacteria are large enough to process the ammonia and nitrite.

In the two weeks it takes _Nitrosomonas spp._ bacteria to become established, the ammonia levels can get quite high. Frequent water changes or the use of an ammonia remover can help alleviate this problem.

A deadly “nitrite spike” follows the rise of ammonia and can last for two weeks until the _Nitrobacter spp._ bacteria catch up. You can avoid this problem by using doses of ammonium chloride to initiate nitrifying activity in the aquarium and not putting any fish in the tank until the bacteria are established.

Use your test kits to monitor this process. Fish are not needed because the bacteria exist everywhere and will become established in the aquarium on their own.

Should the undergravel filter bed become partially clogged with particulates, the water will channel around those areas and reduce nitrifying activity. The result will be rising concentrations of ammonia.

Meanwhile, without oxygenated water passing through some areas of the gravel, anaerobic regions (areas with minimal amounts of oxygen) will develop and become in habited by undesirable bacteria (e.g., heterotrophic bacteria that produce deadly hydrogen sulfide gas, which smells like rotten eggs). If the gravel bed becomes totally clogged, the nitrifying bacteria will die and heterotrophic bacteria will take their place.

There are two ways to deal with this problem:

  1. One is to use a hydro-cleaning device, sometimes known as a gravel “vacuum.” When siphoning water from the tank, the gravel is churned about in the large end of the siphon hose and the particulate matter is removed while the gravel remains in the tank.
  2. The other method is to use efficient mechanical filtration to remove most of the solid material before it settles into the undergravel filter. Clogging is not a problem with trickle filters.

Undergravel and trickle filters are sensitive to power failures. If the filter ceases to operate for more than 24 hours, halting the flow of water through the biological medium, the nitrifying bacteria will begin to die. Once water flow begins again, conditions will be similar to starting a biological filter in a new setup.

Biological filters are also sensitive to some fish medications. In particular, drugs containing erythromycin or methylene blue should NEVER be used in aquariums with biological filters.

Aquarists tend to have strongly held opinions concerning filtration. Some will claim that mechanical filtration and water changes are more than adequate. Others recommend using nothing but an undergravel filter.

They are all correct in the sense that you can get away with any type of filtration for a while. Sooner or later, however, the inadequacies of each filter system will become apparent as fish weaken and die.

For example, if only mechanical filtration is used, DOCs will not be removed from the water and there will be minimal biological filtration from the nitrifying bacteria in the mechanical filtering medium.

This limited amount of biological filtration will be very sensitive to changes in fish load (i.e.,adding more fish or fish growth), changes in feeding rates, and even variations in water temperature.

Moreover, scrupulous cleaning of the mechanical medium will wash away most of the nitrifying bacteria, leading to ammonia spikes. Adding a layer of GAC for chemical filtration of DOCs will not change the fact that there is no effective and reliable method for removing nitrogenous wastes.

In a setup with only an undergravel filter, there is no chemical filtration for DOCs. In addition, without an efficient mechanical filter, the nitrification bed will eventually clog with particulates. Nitrification will be reduced and life in the tank will decline.

In contrast, a filtering system comprised of all three components (mechanical, chemical and biological) provides all requirements for maintaining a healthy aquarium.

The simplest and least expensive setup consists of one outside power filter – either a hang-on-the-back unit or a canister – with separate compartments for mechanical media, GAC and ion exchange resin. It’s important that the filter is large enough to accommodate the appropriate amount of filtering material.

An alternative setup uses two independent filter units. An outside power filter that contains mechanical media and GAC to remove particulates and DOCs is run in conjunction with an undergravel filter.

Although a complete filtration system will cost a little more in the beginning, you save in the long run by avoiding the costs of replacement fish for those that died and medications for fish that become ill.

More importantly, your fish will thrive and your aquarium will be more enjoyable. Contrary to what you might think, a properly filtered aquarium requires less maintenance than one with incomplete or inadequate filtration.

No filtration system, however, can keep aquarium water as clean and healthful as it was from the tap. Over time the water quality deteriorates. Therefore, weekly water changes of 20 percent of the aquarium water are a necessary part of your total filtration system.

Any aquarium store will offer a variety of filters and accessories. These can be combined in many ways. There is, in fact, no single optimum filtration system. The key is to understand the need for a complete filtration system and the principles of how they work.


As a rule, garden ponds that have large volumes of water, lots of plants and very few fish do not require a filtering system. With the proper balance of elements, they can actually be miniature ecosystems. Fish ponds, on the other hand, where the primary emphasis is on the fish, run into the same pollution problems as aquariums. As a result, pond filtration becomes essential.

Pond filtration is similar to aquarium filtration in principle, although it’s a bit different in practice. The amount of water being filtered is often 10 to 100 times larger. In addition, the open exposure of the pond to the outdoors greatly increases the sources, forms and quantities of pollutants. For example, removal of oak leaves and pine needles puts an extra burden on mechanical filtration. The decay of leaves and insects that fall into the pond adds to both the organic and nitrogenous waste load in the water.

Particulate removal in ponds involves filtering out larger particulates and debris. Alternatives to filters include sedimentation basins where large particles simple settle out of the water, gravel or sand beds that remove particles as the water trickles through, and fiber mesh or foam screens.

Most pond keepers ignore dissolved organic pollutants altogether, or count on water changes, which do not really help. The use of GAC for pond filtration is both uneconomical and impractical. The high particulate level and biological activity of the water (e.g., large amounts of suspended algae) rapidly coat the GAC surfaces, while the high organic load quickly saturates the exposed GAC surfaces. A 1,000-gallon pond would require about 26 pounds of GAC and it would probably have to be changed weekly. A foam fractionator (also, and incorrectly, known as a protein skimmer) is quite practical for pond use, but is relatively unknown.

Pond keepers often use ion exchange and bio-filtration to control nitrogenous wastes. Here again, the high particulate and organic loads make ion exchange uneconomic and impractical. Where ion exchange media are used, more often than not ion exchange is not taking place at all! Instead, the media is colonized by nitrifying bacteria, which then remove the nitrogenous wastes. Dedicated pond biofilters, like aquarium undergravel filters and trickle filters, use gravel or specially made media to support colonies of nitrifying bacteria. These are quite effective at eliminating nitrogenous wastes.


If you are using a powerhead to drive your undergravel filter, determining the water flow rate through the filter bed is simple. A powerhead, which sits at the top of the lift tube and pulls water up from under the gravel plate, is rated in gallons per hour. Divide the gallons per hour by 60 to determine the gallons per minute. Then, divide the gallons per minute into the number of square feet of gravel bed to find out if the flow rate per square foot is within the recommended range of 0.5 to 1.5 gallons per minute per square foot.

For example, a powerhead rated at 145 gallons per hour can pump 2.4 gallons per minute (145 divided by 60). An aquarium with a gravel bed 48 inches long by 14 inches wide represents a surface area of 672 square inches (48 multiplied by 14). Because there are 144 square inches in a square foot, dividing 144 into 672 tells you that the biological filter bed has an area of 4.67 square feet. Therefore, dividing the flow rate of 2.4 gallons per minute by the number of square feet, 4.67, reveals that the flow rate is 0.51 gallons per minute per square foot. This is at the bottom of the acceptable range. To increase the rate of flow through the filter bed, a larger powerhead could be used, or another lift tube with a second powerhead could be added, doubling the flow.

Most hobbyists, however, use an air pump to power an undergravel filter. An airstone connected to the air pump by tubing is placed at the bottom of an undergravel filter lift tube. When the pump is turned on, air bubbles flood the lift tube and the air/water mixture rises. As a result, water from underneath the gravel plate is pulled up into the lift tube. Determining the flow rate through the biological bed when using an air pump involves a little extra effort. Check out our reviews for the best and quietest air pumps.

Connect a 90-degree elbow to the top of the lift tube. Most undergravel filter kits come with elbows, but if your did not, you can pick one up at many aquarium stores or use an inexpensive PVC elbow found at hardware stores. Adjust the tank water level so that it is just below the bottom of the elbow lip. Place an empty measuring cup under the lip of the elbow and turn on the pump. Let the cup fill for 15 seconds, read the volume in the cup, determine what fraction of a gallon it is, and multiply by 4 to find the flow rate per minute. Repeat this procedure several times and use the average value. In multiple lift tube systems, test each lift tube and add the values together. Once you have determined the flow per minute, use the procedure outlined above for powerheads to find the flow rate through the filter bed.

For example, if you find that the measuring cup contains 16 ounces of water after 15 seconds, divide the 16 by 128 (the number of ounces in 1 gallon), giving an answer of 0.125 (1/8) gallon. Multiply the 0.125 times 4 to find the flow rate per minute. In this example, the flow rate is 0.5 (1/2) gallon per minute. If there were two life tubes, both with the same flow rate, then the air pump would be moving 1.0 gallon of water per minute.

I have run tests on many air pumps, and I consistently find that maximum total water flow is obtained by using multiple lift tubes rather than just one. The optimum number of lift tubes, however, varies considerably. Too many tubes are just as bad as too few. By repeating the above test procedure using different numbers of determine the setup that works best for you.

After several months of operation, airstones begin to clog and air pump diaphragms start to wear, reducing air flow, and thus water flow. This can reduce the nitrification rate. Regular replacement of the airstones and diaphragms will ensure the optimal performance of your undergravel filter.

Two other things you should know. First, at comparable flow rates, powerheads and air pumps provide the same quality and quantity of filtration. Second, if elbows are supplied with the undergravel filter kit you have, don’t use them during normal operation. They will reduce the flow rate when submerged. For maximum flow, straight lift tubes should come to just below the surface of the water.

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