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Related FAQs: Freshwater Filtration, FW Filtration 2, Freshwater Set-upBiological FiltrationEstablishing Cycling, Undergravel Filtration FW Sponge Filters, FW Canister Filters, FW Hang-on Filters, Chemical Filtrants, Diatom Filtration,

Related Articles: Know Your Filter Media, A Concise Guide to Your Options by Neale Monks, Treating TapwaterEstablishing Cycling, Power Filter Impressions,  A review of some popular mechanical filtration systems by Steven Pro, Undergravel Filtration, Aquascaping, Freshwater Livestock, Freshwater Maintenance

/The Conscientious Aquarist

Freshwater Filtration

By Bob Fenner

Corydoras metae

FW Aquarium Filtration, Principles, Techniques & Tips:

A necessary element of all aquariums is filtration. Without purposeful filtering, aquariums would become "little cesspools" in time. This was not always the case. Many years back, aquarists strived for "balanced" systems, ones with very little animal livestock, sparse feeding, and a good many plants to use up wastes, add oxygen to the water. Modern systems however are almost always over-crowded and over-fed, necessitating the use of mechanical devices to keep their water suitable.

There are many types of filters and a few ways that they work to keep our water clean and clear. As you will see, various kinds of gear and ancillary procedures can be employed to "get you where you want to go"'¦ To the uninitiated it may seem that the disparate means of achieving "clean water" are conflicting and confusing, but with some familiarization with terms and study, you'll soon understand that there are "many roads" to Rome as well as to a viable aquatic environment.


Filtration is the sum total of things we do to change or maintain water quality, adding or removing chemicals, to optimize and stabilize our systems to our livestock's advantage. A great many approaches and types of equipment can be employed to extract and add to system water. Some simple, some more involved, more or less expensive.

In practical terms, filtering our closed systems involves directing the water through physical, biological and chemical media to remove wastes and metabolites like carbon dioxide to sustain the life we keep in a recirculated volume of water. Like so many involved activities in our lives, you can get by understanding "just the basics" here, but the underlying principles are not hard to grasp, and the more you understand, the greater your appreciation and enjoyment will be.


A "classical" classification of processes involved in different types of filtration is presented. These are defined by the actions involved as well as by the ends achieved: Mechanical or physical, biological and chemical.


These include sifting, straining to remove suspended particles such as wastes, uneaten food, algae. Mechanical filters employ media of a small mesh size that restricts these materials passage. Some examples of these filters and media are box filters and their (Dacron) wool, canister filters and various cartridge and pad media, outside hang-on filters and their various sponges and pads. Some mechanical or physical filtration is desirable for use on all systems, but not used to the exclusion of other types, and one must bear in mind the necessity of regular cleaning or replacement of filter media in using strictly mechanical filters.


Biological filtering or cycling of wastes is a necessary series of chemical reactions that convert animal wastes and uneaten foods into less noxious materials. In nature there are groups of micro-organisms that change metabolites like the principal excretory product of fishes and invertebrates, ammonia into mineralized compounds that are useful to other life forms, or out of solution entirely so that they don't accumulate to toxic concentrations. In captive systems it is essential to develop useful populations of these microscopic helpers and circulate the system water by them to prevent this same accumulation <refer readers to the sidebar or separate section on establishing biological cycling>.

Aquarists can elect to speed up the process of establishing nitrifying microbes by using commercial products, "used" gravel and/or filter media, or patiently waiting, but most all-new systems are best approached with sparse initial stocking and feeding, while testing for signs that essential microbial populations are becoming established.

<Excerpt?> The Nitrogen Cycle

One way or window into looking as to what is going on in your aquarium in the way of establishing nutrient cycling is to test for aspects of nitrogen cycling. In an all new set-up there is very little in the way of life, but in a seeming short time (days) various organisms from the air, your hands, other materials placed in the system, will commence virtual wars for space and nutrient, including each other. With or without direction, supplementation by your actions, microbes will come to be situated in and amongst the hard materials of the system that are your allies in preventing biological pollution.

A version of the greatest story ever told (petfish cycling wise)<Need graphics of cycle and chart of concentrations vs. time>

Ammonia (NH3 or NH4OH) is the principal waste product of animals. Unfortunately for us as aquarists it is also toxic in low concentrations in water'¦ and must either be flushed by water changes, chemically removed or prevented in rising to dangerous levels to prevent loss of life.

Happily there are groups of microbes called nitrifiers that chemoautotrophically convert ammonia to nitrite compounds (NH3 > NO2), an aerobic (using oxygen) process'¦ and over a period of days to weeks these microorganisms increase in number and dominance to aid in animal waste conversion in our aquariums. Unfortunately, nitrites are also toxic to aquatic life and it falls to another group of nitrifying microbes in time to convert nitrite to nitrate (NO2 > NO3) which is far less toxic than ammonia or nitrite.

Nitrates can become troublesome if allowed to accumulate to high concentrations (several tens of ppm.), possibly causing algal blooms and poor overall health, but these compounds can be diluted through water changes, taken up by chemical filtrants, demineralized by anaerobic processes, utilized by photosynthetic life'¦ reduced in overall concentration in several ways.


Selectively removing materials down to the molecular and atomic level is the purpose of chemical filtration. Activated carbon, synthetic resins, Zeolite and other clays and more are employed in these regards. In practice, water is directed through these media (which are best contained in a bag) and these media periodically replenished/replaced as they become exhausted.

Some folks use chemical filtrants on a regular basis, as incorporated elements in their filter equipment, others just use them from time to time, as when their water "yellows" or they want to remove a medicine treatment. Most aquarists skip using chemical filtrants altogether in time, relying on regular water changes and other maintenance, along with their pre-treated make-up water to keep their water about right.

The use of chemical filters in producing new aquarium water is such an important and involved topic that we have discussed this elsewhere <refer to Treating Your Tapwater for Aquarium Use>


There is likely no other aspect of aquarium keeping more controversial and rent with opposing opinions than filtration. Know that the ends of all filtering is the same: to provide clean, viable water to our living charges, AND that the means of doing so are many and various. All types of gear has its admirers and detractors'¦ some is relatively simple, others more involved. Depending on your budget, types of livestock, goals as an aquarist and time and trouble you want to put into checking, working on your system, you will find you have several choices of how to go about filtering your aquarium/s.


Thinking through what sorts of livestock you intend to keep, how much time you'll want to put into keeping your system properly, and what sort of budget you want to apply will pay huge dividends later on in enjoying your aquarium. Know that there is a very wide spectrum in the "dirtiness" of different livestock, their foods, feeding, that different types of filter gear can save you on time and labor or be a waste of time'¦ and money. The most expensive gear may not be the best, and all aquariums require regular maintenance'¦ and time.


A few years back one of the industry manufacturer's conducted a study to determine the public's attitudes towards the aquarium hobby. They found that the single-largest negative impression people had toward our interest was that it was "drudge ridden", a bunch of work to maintain. For folks who have taken the time to plan and build their set-up properly, such activity is part of the fun and enjoyment of aquarium keeping. Indeed, regular upkeep of our tanks need not be overly time-consuming, messy or laborious. With some simple tools, the routines of cleaning, changing water, testing, recording and observing our systems is a breeze.

Developing a system of tasks on a regular and periodic time schedule can aid all in keeping track of what needs to be done.

Daily: While feeding your livestock, looking over all your livestock, equipment to make sure all is in order is a good idea. What is the water temperature? Are all livestock present, feeding? Are all pumps in working condition?

Weekly: Activities include such items as "getting into the tank". You may want to employ a dedicated set of arm-length gloves here to prevent contamination. Tank insides and out are cleaned, gravel vacuumed and water changes made, small changes to decor executed, live plants pruned if present'¦ Regular water testing is done at these times and recorded. Filter media should be checked and cleaned or replaced as necessary. This is a good interval to check electrical connections, timers if used, and tubing connections as well.

More Periodic: Include more thorough clean ups, perhaps changing out part of the gravel substrate, or re-doing the plantscape and/or rockscape of the system Other items like light replacement, pump cleaning and tune-up, larger scale remodeling are included here.

For folks so inclined an "Aquarium Log" book, or pages in a folder on your computer can be devised to record your activity and observations. There are even handy pre-made tools of this nature available for your use.

A Review of Filter Gear/Types:

Box Filters:

Before the 1960's these were the ubiquitous tools of aquarium filtration, employing fiberglass "wool" and charcoal usually to act as both mechanical and biological filtration. This venerable tool is still with us, though not as popular as other means of filtration due to its "clunky" appearance and the room they take up

Undergravel Filters:

Though the popularity of under the gravel filters has drained over the years, their use continues. These are devices that capitalize on the oxygen-using microbes mentioned above, drawing food and wastes into the rocky substrate on the bottom either through airlift or powerhead means, and having the life there break it down out of sight. Undergravel (UG) use has its drawbacks however, as speeding up nitrification can result in too rapid accumulation of nitrate, and UG systems tend to accumulate a good deal of "dirt" under them, resulting in more-complex clean-outs. Nowadays, aquarists tend to use other means in addition to UG if they use these tools at all.

Power Filters:

Not so much a type of filtration per se as a means of actuating water movement, power filters of different sorts, hang-on, canister, inside'¦ use fluid-moving pumps to push or pull system water through filter media, where it is processed mechanically, biologically and perhaps chemically. <pix of examples of these types of filters. RMF suggests we solicit the manufacturers for graphics, or if necessary products for this purpose> These powered filtration devices can be very useful tools.

Other Types of Filtration, Gear:

Sponge filters, ultraviolet sterilizers, ozonizers, contactors of different sorts'¦ and much more are part of the panoply of gear one will find as you get more involved in the hobby. All types have their place and uses, and their concurrent bother and maintenance.


    Filtration embodies all we do to maintain water quality for our living aquatic charges. Mechanical means serve mainly to remove particulate matter, biological filtration involves the expedient cycling of wastes to less noxious compounds, chemical filtrants aid in further removing metabolites and color. There are many "roads" to the desired goal of effective aquarium filtration, given choices dependent on your stocking selections, sophistication and monies to invest in acquisition and operation. You are STRONGLY encouraged to investigate your options thoroughly, with an eye on the very real possibility of what role your gear purchases might play in future upgrades to other, larger systems. Do look into what others have to say re prospective purchases, at your LFS's, the various Bulletin Boards on the internet and hobby magazines. Look for long-term success reports by others, including ease of maintenance issues, and weigh your options carefully in terms of electrical consumption and replacement of filter media.


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