There is a good variety of equipment in multiple formats sold for aquarium filtration, most of which will do the job when properly used. But there also seems to be more than a little confusion among those who are new to the hobby, or who lack the information helpful to understand exactly what these devices are doing in their tanks. So I would like to offer a series about the principles of filtration, hopefully to give the user a little better understanding of what these machines are doing and how they do it. I’m not going to try to suggest a list of best or preferred brands filters for any given use (although I will mention the particular filters I use). But I will try to give a little information on the jobs that each type does well, and you can make your own selection from the brands available. This information is fairly technical, so somewhat heavy going. But I hope it will provide a deeper understanding that will help hobbyists.

If you are a careful reader, you noticed that I said “machines” in the first paragraph – intentionally. I’m not going to review filterless tank techniques here (such as Diana Waldstad advocates, or other form of “natural” tanks), nor detail specialized processes such as plenums, or plant or “veggie” filters (for more information on the last, see: Veggie filters article

So this will be about machines or devices (whether electric or air-driven) that produce some sort of water flow, which in turn provides filtration, and how that filtration occurs.


The water flow produced by these devices is a given by the definition we are using. Current itself contributes to filtration by keeping particulates in suspension for capture by the filter media (1). It also provides currents of some force, which may or may not be desirable depending on:

  • The amount of the current produced
  • Where the current is produced
  • Whether or not the tank is planted and how heavily it is planted
  • The individual needs of the fish being kept.

It is becoming a cliché, but is a valid point, that it is not possible to over-filter a tank. Mechanical filtration is open-ended, restricted only by the particle size of the material to be captured. The more filter material or medium you have, the more you potentially can capture. The finer the particle size captured, the more material is captured. Adding more mechanical filtration medium beyond the minimum adequate amount, or better, the optimum amount, affects the cleaning interval more than anything else. Longer intervals may or may not be desirable. Other factors do enter that area and are discussed later in more detail.Chemical filtration is both material-limited and concentration limited. First it is limited by the particular substrate or medium employed to capture dissolved materials, and second by either the concentration or the total mass of each dissolved material to be captured. Chemical media has volume and cost restrictions, and practicality issues similar to mechanical filtration.

Biological filtration is limited by metabolite concentrations in the water processed. Biofiltration performs best where there is ample flow of prefiltered water to provide the bacteria with oxygen and nutrients, including both un-oxidized metabolites (ammonia or nitrite) and other minerals and nutrients. Biofiltration needs or requires protection from silting and suffocation by collected particulates, but beyond certain minimal attachment area requirements, more surface area alone will not give more functional colony sizes than the waste produced requires. Colony size is determined and limited by the rate of in-tank production of un-oxidized nitrogen metabolites much more than any or factor, but also by oxygenation and flow.

Too much or too little current is possible and either extreme may be undesirable. Current aids oxygen exchange at the water surface. Water holds significantly less oxygen per unit volume than does the air above the tank, and the absolute quantity of oxygen that water can hold decreases as the temperature of the water increases – i.e., cooler water can hold more oxygen, warmer water less oxygen. Carbon dioxide is produced by the fish (just as it is by people, but land-dwelling animals release the gas, fish release the dissolved gas) and is released into the water in solution. This dissolved gas can escape to the air at the water surface. Aerobic (oxygen-using) bacteria also use oxygen and release carbon dioxide. Actually, so do plants use O2 and release CO2, but healthy plants release more O2 during photosynthesis than they use themselves in 24 hours. So the two gases of particular interest here, oxygen (O2) and carbon dioxide (CO2) exchange readily at the surface, O2 in and CO2 out, usually, but not always (2).

Dissolved ammonia is far too soluble to off-gas easily under normal tank conditions and concentrations. The oxygen-enriched, carbon dioxide-reduced water will be circulated through the tank by the current from the filter or other devices. The amount of oxygen dissolved in the water may be an issue depending on the fish and other bioload in the tank. The current provided by the filter may be the primary process for circulating the tank water to pick up oxygen at the water surface. With the selection of motor driven filters currently available, if the filters are adequate to keep the tank clean and are well-maintained, aeration should not be an issue. This means that the use of airstones for aeration (3). is more of a question of the hobbyist’s aesthetic choice than to meet tank’s needs for oxygen exchange.

Current also avoids temperature stratification in the tank. At aquarium temperatures, the warmer water would be near the surface and cooler water near the bottom. Other than in some planted tanks, temperature stratification is not a big issue in hobby tanks. Some planted tanks with little current in rooms with a significant difference (more than 10-15 degrees F) in temperature between the ambient air and the tank would be most likely to show stratification. Most fish would be completely untroubled by the gradients that would be likely in most hobby tanks, but with current these should be all but undetectable.

Planted tanks lacking sufficient current (and/or with very thick substrates) in such rooms may suffer due to the effect of “cold feet” on the plants. The substrate, as it has only diffusion rather than current through it, can lose heat to the cooler room. The net effect is root zone temperatures several degrees below the tank median temperature. I have no personal experience with this, but several plant people assure me it not a good thing. There have been several technologic “cures” for this condition, but none have been proven effective and beneficial in general use.

Important Note: Aquarium filters do not and cannot remove things from the tank. This is basic, and needs to be more widely understood:

  • Filters may trap particles (mechanical filtration).
  • Filters allow for change of metabolites (products of metabolism, frequently but not always waste products) from one dissolved form to another (especially from toxic to less toxic, biofiltration).
  • Filters may trap and hold certain materials, but they do not remove them (chemical filtration).

Out of sight must not be out of mind. The materials captured or modified by the filter devices are still present in some form in intimate contact with the water until the hobbyist removes them. Sometimes these materials are unlikely to be returned to the tank water, as is generally the case with chemical filtration. Sometimes they will continue to contribute to water pollution as materials dissolved in the water, whether they are visible or not, as by mechanical or biofiltration. The filter devices we use make our job of removal easier, but alone they can do only part of the total job needed, we must do the rest. Collected or adsorbed materials are removed when we do periodic maintenance on the filter. Dissolved materials are removed when we do partial water changes on the aquarium.

If you remember nothing else from this article, this is the key point, the primary take-home lesson on filtration – filters collect and/or modify material, aquarists remove and many individual filter units or formats which will adequately filter any given tank. Any appropriate filter or filters selected should have a margin of safety, a certain degree of over-capacity. But simply increasing filter volume or turnover does not in and of itself increase the total carrying capacity or stocking level of the tank. Maximum carrying capacity or best stocking density are not a simple matter of filtration in most hobby tanks.

Many factors are involved in stocking, not just filtration.We can start by dividing filtration into at least three broad categories whose names at least should be familiar: mechanical, chemical, and biological filtration. These are not always as easy to separate in practice as they are on paper, and all three may be performed within a single filter device, or may divided among various separate devices.

Mechanical Filtration:

By definition we can say that this is physical removal of particulate material (undissolved, suspended particles) from the water column, capturing materials (particles or particulates) so that they are no longer free-floating in the water column. This material captured is not dissolved in the water, only suspended. It is made up of particles of whatever size – from individually invisible (“cloudy water”, either green from algae or gray from bacteria), to flakes of food, plant leaves or fragments broken away from the parent plant, bits of fish poop, or even dead fish. These particles are usually trapped by porous material that may be floss (threads or fibers as bulk loose material or formed into mats), sponges or grids, fused or foamed glass or ceramic, microporous sheets, gravel, sand or smaller-grain material, or even an otherwise empty vessel, i.e., a settling tank, with or without baffles.

The last item listed above, a settling or sediment chamber, is seldom used in hobby tanks. It would involve use of some water-holding vessel such as an empty tank, with water from the tank fed in at one end, usually with a number of current-damping baffles along the length, and an outlet (which may be a pump) at the opposite end. Particles above a certain size and density will settle out from the lack of supporting current. This sediment can be removed by vacuuming, or if the vessel has a properly shaped bottom fitted with a drain, simply by opening a valve.

Normal use of settlement/sediment filters is in highly stocked outdoor ponds with koi or other high waste producing fish. Sumps connected to tanks might serve this function if arranged to damp out currents. Excessively large grain gravel substrates also serve as a sedimentation area inside the tank itself, and are particularly difficult to maintain. That particular situation is one to be avoided.

See : Using the Python Products’ “No Spill Clean and Fill” for more discussion on that issue.

This is a good point to introduce another concept: effective pore size. This would be the average size of the smallest particles that would be trapped by the medium, would be held and not pass through along with the water back to the tank water column (4).. Mechanical filter materials come in a relatively enormous range of pore sizes, from a fraction of a micron (one millionth of a meter) to significant fractions of or even whole centimeters (one hundredth of a meter, about 0.4 inch). Remember that most tank filtration is “pass-through”, that is, the material being filtered (tank water) must pass through (5). the filter medium to have its particles removed or reduced. More of this idea will be discussed later, but for now just hold onto the concept that any arrangement of particulate-trapping material will have an effective pore size, whatever the scale.

Starting at the smallest end of the scale with micron filtration (diatom powder and microporous membranes), we can capture not just visible suspended debris, but infusoria such as rotifers and protozoans, and many bacteria or unicellular algae (cloudy water and green water, respectively). Diatom filtration works quite quickly relative to other forms of filtration, as it begins at the single-digit micron range of effective pore size.

This and any other pass-though filter media tend to become more effective filters and to reduce their effective pore size as they operate. If a diatomaceous earth coated filter is just staring up, the effective pore size is likely to be a very few microns. After it has been running only a few minutes, it will have picked up quite a number of particles of sizes larger than the pass-through measure. These still-small particles are gathered around and inside the openings that they are just barely too large to pass through, and held there by the current flow. The buildup of these trapped particles will reduce the opening’s effective pore size, so smaller particles will start to build up atop and around the original layers, even though at first they passed through the filter easily. Soon the effective pore size will be under a micron, and bacteria and such will be trapped on the filter with the larger particles.


Now we are really filtering the water! Bye-bye cloudy water, so long pea soup! OOPS- the filter is clogged, even water cannot get through anymore, or the flow is reduced to a trickle. A basic paradox: the finer (smaller) the pore size of the filter, the faster it clogs and ceases to function. This occurs in part within the medium itself (in this case inside the diatom powder layer). The “pores” are not at all neat cylindrical openings channeling through the depth of the layer of powder. Diatom powder is the silicate skeletons (external in the living organism) of diatoms. Remember diatoms? Brown algae? Slightly gritty feeling brown slime in the darker areas of new tanks? Yup, that is the stuff. Deposited by slow buildup in shallow seas in the remote past, these geologic strata are made of the cast-off intricate cases that once housed these oddball photosynthetic organisms.

To step into the macro world for a large-scale illustration: Picture the plastic eggcrate light diffuser grid used in several DIY tank techniques. Imagine a cut circle of this material that would be sealed into the joint between two diameter sections of large drainage pipes (for our purposes standing vertically). This grid would represent the bag or membrane on which the diatom powder itself will be captured. Now pour a large bag of “shooter” marbles into the upper pipe, making a layer several to many marbles thick on the plastic grid. The shooters represent our macro version of diatom powder. The pores are the openings and channels between the marbles. These present a torturous path between the component marbles (or the diatom cases and fragments they represent). Slowly pour in a mixture of regular marbles, aquarium gravel and sugar sand.

In this example, with the regular marbles representing routine (visible) debris, the gravel could be bacteria and algae; the sugar sand could represent the water molecules themselves. When we first start adding the mixture, the sand goes right through, as does some to much of the gravel, but the regular marbles (representing larger tank debris) build up more layers on the surface, making the effective pore size smaller. As this layer develops, the gravel (representing bacteria or unicellular algae) starts building up as well, both on the surface and within the channels. The regular marbles were our ordinary particles, which in their turn produce a finer filter. As the fine stuff builds up, the same happens with the gravel (representing bacteria) and even the sugar sand – representing the water molecules – will be stopped.

Our macro “filter” model is clogged. All of our aquarium filter media follow a similar pattern in our tanks, whatever the absolute scale of their particle capture. When brand new or well cleaned, their capture is not as good as it will be later in the course of capturing materials, of filtering, and eventually the material will partly or fully clog. The filter unit’s water output will slow or stop, unless the water flow bypasses the mechanical medium.

Diatom powder could be considered the most efficient, effective form of mechanical filtration because it holds even particles individually invisible to the naked eye. The water after diatom filtration will be mountain-lake clear, which is quite noticeably different from the water clarity with “normal” filtration. We are accustomed to seeing our tanks with a significant density of suspended particulates. These particulates hopefully are individually invisible, but collectively they do reduce the clarity of the water. But, and a very big BUT, the diatom filter clogged too fast.

Nobody is going to clean their filters several times a day (in practice, it would not be that bad, as once the majority of the particulates are gone, the filter clogging will happen much more slowly). This is another basic on aquarium filtration – compromise is needed for practicality. We will select devices holding media that will do the job we want, day in and day out for some acceptable period of time without clogging, and we will clean or replace the media ourselves before it is fully clogged and start over. We start with media of larger than optimum porosity, knowing it will become finer in use. Remember the note early in this article? The filter did not remove the particulates from the tank; it captured them so we could remove them when we clean the filter.

So we compromise, we let some particles pass through the floss or sponge (the smallest), but we capture a lot. We keep an eye of the water return and we clean the filter medium before the water flows slows too much. That is the common practice for maintaining mechanical filters, and was my own for years. Now I try to do a little bit better. Mechanical filtration captures lots of organics in particulate forms – fish poop, uneaten food, plant debris, etc. Why should we leave this material in the tank longer than necessary? Organic material such as these particulates trapped on or in a sponge or floss is ideally situated to be attacked by infusoria, heterotrophic bacteria, and fungi, and is digested by these creatures. The soluble byproducts or wastes from such digestion is returned to the water column as dissolved materials, some organic, some inorganic.

I have as much as possible separated my mechanical filtration so that I can rinse it at least weekly. Primarily I use sponges, as they are easy to rinse and the material lasts through many, many rinse cycles. I remove these from the tank, rinse under tap water (this is mechanical filtration, I’m not concerned with nitrification bacteria here) and return the sponge (or floss, or fiber mat, all are the same principle) to use. In this way I can get at least some reasonable percentage of the particulates out of the tank before it is broken down completely and would return its organics to the tank water (dissolved organic compounds, DOC, may be significant factors in tank pollution) along with dissolved nitrates and assorted other dissolved minerals. Daily rinses would be far better, but that will not happen in my tanks.

In fact, I learned this in my salt-water days, being taught by a pro to rinse the prefilters on my wet/dry filters daily – it worked really well, but I am too lazy to do this for FW – compromise again. Every other day or at least twice a week would be really good, but that is unlikely for me as well. Every week is twice as good as every two weeks, four-plus times as good as every month, eight and half times as good as every other month. I do remember what those sponges looked like after a month, or two months – brown sludge city. No thank you, and thanks to this habit, my nitrates (which are easily measured with hobby kits, where DOC is not readily measured at home) are much lower now.

How do you set up for mechanical filtration? Hang-on (HOT – hang-on-tank, HOB -hang-on-back) power filters do a good job, providing good surface breaking (i.e., gas exchange) at the same time by their returns. The last item is not great for planted tanks (it blows off the elevated CO2 concentrations needed in many planted tanks), but is excellent for aerating (3) fish-only tanks. Surface disruption can be controlled in part by the tank’s fill level: maximum fill equals least surface break; minimum fill gives the most surface break and depth of current in the tank.

Most commercial units are quite easy to service in that the bags or sponges are simple to remove and clean or replace as required. With some of these units, filter media by-pass is possible. This means only that water can flow around or over the media, especially dirty media, rather than through it (pass-through is required, remember?). So keep an eye on your filter media to be sure it is clean enough that most or all of the water passes through the media rather than around or over it. That last case would be circulation without much if any mechanical filtration – in other words, the pump is working but the filter is not.

External canister filters can do an excellent job with their controllable returns (you can set for the amount of surface agitation and current desired by position of the return – a topic in itself) and offer quieter to completely silent operation, but are not the best equipment to open every week to rinse the sponges. For canisters I prefilter the intake with a sponge (6). so it can be pulled for rinsing without disturbing the canister itself. These sponges must be kept clean to avoid starving the unit for water flow. The external canisters I use (Eheim) have suffered no ill effects from extended use (decades) with regularly rinsed sponge intake prefilters, but I cannot discuss other brands, as I have no experience there.

These must be kept clean or even the best filter would suffer at least flow starvation (and loss of function due to reduced flow) if not damage to the motor itself – cavitation (7). in the impeller housing can be a symptom. I do not rely on those sponge prefilters for all of the mechanical filtration in the tank, but provide at least one addition filter and commonly multiple units for mechanical-only filtration. This could be a HOB, but for me is usually an internal canister (quieter and more controllable current), but it could also be a simple air-driven or sponge-and-powerhead combination. The object here is to avoid my having to open an external canister to get to the mechanical filtration medium frequently for cleaning. If I make it easy to do, I am far more likely to do the routine tasks when they are needed, not just when they absolutely must be done.

There is a big difference in the water quality of tanks maintained on an absolute requirement basis (this filter is bypassing, this canister is almost stopped) and those maintained on a best-practice basis using pre-set intervals shorter than significant detectable decrease in flow, or “that sponge looks dirty”, or that “this canister’s flow has dropped”. These issues are personal options. I want to provide the best tank conditions that I can. But I also am unwilling to do time-consuming labor-intensive tasks frequently. I like a middle ground where the frequently needed chores such as mechanical filter rinsing are as easy and quick and relatively mindless as I can make them. I do know myself well enough to know that only in that way will those individually easy tasks get done.

Air or powerhead driven sponge filters have similar upkeep requirements, but may call for greater care in rinsing if they are used for biofiltration as well as mechanical filtration only. Generally that calls for rinsing in removed tank water or other aged or disinfectant-neutralized water of similar temperature and water parameter profile to the tank water in order to protect the nitrification bacteria.

Several experienced tank-keepers use HOT Magnum canisters with the available microporous membranes as fine mechanical filtration (Mighty Pleco/MP/aka MP started or first popularized this practice so far as I know). This supplements other, routine effective pore size filters, also frequently maintained. Assuming the tank is already well filtered for the bioload, adding this unit can, by reliable reports and photographs, keeps the tank at after-diatom filtration clarity routinely. The maintenance needed is weekly trading out of the microporous membrane for cleaning (at least two microporous membranes are stocked, one in use in the unit, the other being chemically cleaned by bleach solution). I am tempted, but have not yet gotten hooked on this near-ultimate mechanical filtration practice.


Mechanical filtration is to remove particles from the water column, so that our tank water appears clear. It does not remove them from the system; we have to do that by cleaning the filter. If we clean the mechanical filter as frequently as is practical for us, we will reduce the pollution load in the form of DOC and nitrate and other materials dissolved in our water (improved water quality) along with visibly improving the water clarity.

Footnotes and Clarifications:

1. “Media” is the plural form of the word. “Medium” is the singular form. These are the materials that physically trap particles as mechanical filters, or the materials that absorb (captures and holds materials within its own mass) or adsorb (captures and holds materials on its surfaces).

2. Normal oxygen use reduces the absolute quantity and the concentration of oxygen in the tank water below that expected from equilibrium with the air over the tank, so the ordinary exchange is for oxygen to move into the water. The opposite is true with carbon dioxide in fish-only tanks, normal respiration increases the CO2 levels, so excess over “air-equilibrium” or the normal balance with the air, will have CO2 moving out.

Do remember that gases in solution are independent of one another. That means that the oxygen concentration is independent of the CO2 concentration, or the N2 (nitrogen gas) concentration, or that of the other dissolved gases present. Air is a mixture of gases. In solution in water, each gas is independent. Oxygen gas by its nature is less soluble than CO2, but it is present in the air at many times the concentration of CO2, so the same relationship, more dissolved O2 than CO2 in water, still holds. But the solubilities are quite different.

In planted tanks without CO2 supplement, CO2 may also move into the water from the air as the plants use it, which reduces water concentrations below air-equilibrium levels. That allows more CO2 to dissolve.

In planted tanks with bright light and CO2 and other supplements, during the light cycle the plant produce more O2 than they can use. They release the excess to the surrounding water. The tank may become saturated with O2 (meaning no more O2 can dissolve), or even supersaturated. In supersaturated O2 conditions, excess O2 appears as tiny bubbles released by the plant leaves, as no more O2 will dissolve. These bubbles release the excess O2 into the air at the water surface. This is referred to as “pearling”.

3. Aeration means only adding air, or more specifically, oxygen to the water. Long ago, filters were far less efficient and moved far less water than those we use now. Airstones had a functional place in the hobby. Current filter construction and design is likely to provide quite enough filtration and aeration without the need for airstones. That statement does presume that the filter is properly sized for the setup, and properly maintained. Airstones today are more décor than functional requirements in conventional tanks.

Airstones do the job that was once needed by the vertical and horizontal currents that they set up in the tank. The column of bubbles and water lifting from the base of the water column, spreads out parallel to the surface, and establishes a circulation in the tank. Gas exchange takes place in the surface exposure provided, much more than from the bubbles themselves. Also to some extent, gas exchange is aided by the surface breaks made by the bubbles in breaking at the surface.

4. The “water column” is just a two-bit term for the visible water in the tank, i.e., the water between the substrate and the water surface level. This is not the entire water volume of the system. There is water held in the substrate (which exchanges with the water column by diffusion only, other than with the use of UG, RFUG, or heating cables), and in any external filters or refugia (HOB/HOT, canister, sumps, etc.) exchanging with the water column by siphons and pumps.

5. “Pass- through” filtration media has effective pore size substantially larger than much of the material to be filtered, at least initially. In tanks we are dealing with water, so the effective pore size of new or clean medium is always many time the size of a water molecule. The material to be cleared or cleaned, the water, passes easily through the filter medium, while at least some particles are captured and held by the filter medium.

6. See “Biofiltration-Only Canister Filters”

7. Cavitation is the phenomenon of air or gas bubbles being forced or sheared from the water itself by a stagnating impeller or other drive device. There are always gases dissolved in water in open contact with the air. When a simple flow pump is intake-starved, the moving (without being able to normally move water) impeller causes bubbling. The type of pumps we use in most of our filters can have the output restricted to some degree by head pressure or by valving down, but should never be intake-restricted. Cavitation as a symptom should not be ignored. Damage to the drive may occur.

NextPart 2, Chemical Filtration.

Robert T. Ricketts, a.k.a. RTR


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