Basics of Filtration

 

Aquarium Setup #4 – March 1993

Basics of Filtration

 

by Dr. Steven J. Schiff

 

            Life is full of choices.  In fact, much of life consists of making decisions and living with the conse­quences of those decisions.  For example, we have all made the decision to become aquarists.  Having done so, we find ourselves faced with myriad further choices to make: Fresh or salt water?  Fish or inverte­brates?  Flake food or pellets?  Shall I do a water change or lie on the couch, watch TV, and think about doing a water change?  (Some choices are easier than others.)  Each of these decisions, of course, leads to endless new ones (for example, what show should I watch while I an lying on the couch thinking about doing a water change?)

            This article is intended to make one of your choices easier.  The decision to be made, in this case, is what filtration to use in a given aquarium.  While I can’t make that decision for you, I can provide information that will permit you to make an informed choice within the constraints imposed by your own particular situation.

            A very broad definition of filtration is “the treatment of aquarium water to make it more liveable for fish.”  We can add to this by saying that we must pass the water through some kind of device to perform the treatment.  This column will help begin­ning aquarists understand the basics of filtration and review the fundamental principles of aquarium filters, as well as some common products that incorporate these principles.

            We generally speak of three major types of filtration: mechanical, chemical, and biological.  Most filters provide at least two of these types, and some provide all three.  Different filter types are better at some types of filtration and worse at others.  Our task as aquarists is to select the appropriate equipment to do the desired job.

                                                              Mechanical Filtration

             Mechanical filtration is what we usually think of first when we talk about filtration–this is what keeps the water “clean”.  Most filters provide at least some mechanical filtration, but again, some do it better than others.

            In most aquaria, outside power filters are the best type for mechanical filtration.  They provide a high water flow rate, which circulates the water thoroughly and thus removes solid waste material before it can decay and foul the water.

            For efficient mechanical filtration, we need at least two tank turnovers per hour – that is, the entire volume of water in the aquarium should pass through the filter at least twice per hour.  This is really a bare minimum – we are much better off with four or five hourly turnovers, and even as many as ten is not excessive in most cases.  (Instances in which this much water flow would be excessive include tanks of fry or slow-moving fish, such as long-finned fancy goldfish.)

            No common filter types other than outside power filters provide such a high flow rate at reasonable cost.  Other filters, such as inside power, canister, sponge, and box filters, also perform mechanical filtration, but their smaller size and/or lower flow rates tend to make them less effective.  Therefore, they should be carefully matched to the tank size and particular application for which they are to be used.

            The diatom filter is a special type designed for mechanical filtration only.  The filter medium is diatom powder, which consists of the skeletons of microscopic algae-type plants.  These form an extremely fine sieve which can catch particles as small as one micron (one millionth of a meter, or a thousandth of a millimeter).  The filter thus removes all particulate matter larger than bacteria, and produces extremely clear water.  Because of this extremely fine filtering capability, diatom filters clog quickly and are not intended for continuous use.  Rather, they are employed occasionally to “polish” the water and remove the free-swimming larval stages of parasites such as Ichthyophthirius (“ich”).

                                                                Chemical Filtration

             Mechanical filtration removes particles from the water, but doesn’t change its chemical makeup.  Chemical filtration actually changes the chemical characteristics of the water.  We do this for two reasons: 1) to remove toxic substances or waste products from the water, and 2) to alter the water to make it more suit­able for a particular type of fish we want to keep.  For example, if we wish to raise discus or neon tetras (which require soft, acidic water) and our tap water is hard and alkaline, we can use chemical filtration to soften the water and lower its pH.

            Chemical filtration usually involves passing the water over a medium which alters its chemistry.  Some such media include:

            • Dolomite or crushed coral - increase hardness and pH.

            • Zeolite - removes ammonia.

            • Activated carbon - removes organic substances, medica­tions, dyes, etc., via adsorption onto its huge surface area.

            • Peat - decreases hardness and pH.  Also adds beneficial substances to the water, such as tannins and humic acid.

            These substances are placed in a filter, and the water is altered as it flows over them.  Canister filters are particularly good for this purpose, as they will hold more filter media than most other types.

            Another, more specialized type of chemical filter is the foam fraction­ator, or “protein skimmer”.  This device works almost like activated carbon, removing organic substances, dyes, medications, etc., by adsorption onto the surface of enormous numbers of very small bubbles.  As with activated carbon, surface area is the key.  Foam fractionation works better in salt water than in fresh water; as a result, these devices are usually found in marine aquarium systems.

                                                                Biological Filtration

             Biological filtration is probably the most important of the three major types of filtration, as well as being the least understood.  It is not difficult to accomplish.  It involves growing as large a population of beneficial nitrifying bacteria as possible by providing the proper condi­tions.  These bacteria live by breaking down certain nitrogenous waste products produced by the fish; in doing so, they convert these toxic substan­ces into less harmful forms.

            As shown in Figure 1, the first step in biological filtra­tion is the breakdown of ammonia into nitrite by bacteria of the genus Nitrosomonas.  Ammonia is excreted directly into the water from the fishes’ gills.  It is also a breakdown product of their urine, solid waste, uneaten food, and any other nitrogen-contain­ing organic materials in the tank.  It is quite toxic to fish, but a properly functioning biological filter can break it down as fast as it is produced.

            The product of the breakdown of ammonia is nitrite.  Unfor­tunately, this is still rather toxic to fish.  Fortunately, the other bacteria in the biological filter, genus Nitrobacter, use nitrite as food, and break it down into nitrate, which is rela­tively nontoxic.  The concentration of nitrate in the water can go quite high before affecting the fish, but it eventually does cause problems.  For this reason, we perform regular, partial water changes, which dilute the nitrate and keep its concentra­tion at safe levels.  I don’t think we can emphasize the impor­tance of water changes enough.  Failure to do them probably results in more failures in keeping fish than any other problem.

            Nitrobacter and Nitrosomonas bacteria do not float freely in the water, but must attach to a solid surface.  The purpose of a biological filter medium is to provide as much surface area for bacterial attachment as possible, while allowing free flow of water over the medium (i.e., not clogging).  Some examples of biological filter media include:

            • Filter floss – very large surface area.  Inexpensive, but clogs easily.  When discarded, the bacteria living in it are discarded too, reducing the effectiveness of the biological filter.

            • Sponge or foam – good surface area.  Reusable and inexpen­sive.  Rinsing in aquarium water outside the tank will remove dirt without signifi­cantly impairing biological effectiveness.

            • Woven nylon pads – good surface area.  Reusable and easier to clean than a sponge; less likely to clog.

            • Ceramic rings or “noodles” – moderate surface area.  Heavy but extremely durable.  Don’t clog easily.

            • Specialized “biological” media – included in this category are all the products intended specifically and exclusively as biological filtration media, such as BioChem Beads^R and Stars, BioBalls^TM, Bio-Blocks^TM, Bio-Pak^R, etc.  There are many compet­ing products in this category, and a lot of advertising claims are made for them.  In evaluating these claims, we must keep in mind the basic purpose of the medium and use the information presented here and elsewhere to make an informed decision.

            Nitrification (the stepwise conversion of ammonia to ni­trate) is an aerobic process, which means that Nitrobacter and Nitrosomonas require oxygen to perform their functions.  This is why so-called trickle or wet/dry filters work so well.  They provide, better than other designs, the three elements that are essential for growing these bacteria:

            1) Large surface area (these are the largest filters gener­ally used on home aquaria);

            2) Constant, gentle flow of nutrient-rich water - the prolonged contact time of the water with the media allows the bacteria to completely remove ammonia and nitrite; and

            3) Water flowing over the bacteria is saturated with oxygen.  In a wet/dry filter the media are not immersed in water, but rather are coated with a thin film of it as it trickles over them.  This is enough to keep the bacteria wet, and the close proximity of air ensures that dissolved oxygen is at a maximum.

                                                             Common Filter Designs

             As I mentioned earlier, each type of filter performs differ­ently with respect to the three major types of filtration.  Choosing a filter for a given aquarium requires knowledge of the strengths and weaknesses of each type.  For example, a tank set up with both an outside power filter and an undergravel filter would benefit from the excellent biological filtering performance of the undergravel filter, as well as the superior mechanical filtration effec­tiveness of the power filter.  Combining filtra­tion methods in this manner is a superb technique for custom-tailoring a particular installa­tion to precisely meet its re­quirements, while using readily-available existing products.  The following guide rates some familiar designs with respect to biological filtration ability (BFA), mechani­cal filtration ability (MFA), and chemical filtration ability (CFA):

             • Box filters are simple and basic.  Filter floss provides mechanical filtration and surface area for biological filtration.  They can be filled with chemical media such as carbon, crushed coral, ammonia-removing resins, etc.  These are good all-around filters for small tanks.

            BFA: Good.

            MFA: Fair (flow rate too low).

            CFA: Good.

 

            • Sponge filters are excellent for fry tanks.

            BFA: Excellent (high surface area).

            MFA: Fair (flow rate too low).

            CFA: None.

 

            • Canister filters are extremely versatile.

            BFA: Excellent.  Can be filled with almost any medium or combination of media.  The prolonged contact time between the water and the media and the high media volume promotes complete nitrification of ammonia.

            MFA: Fair to good (depends on flow rate).

            CFA: Excellent, due to the large media volume and wide choice of available media.

 

            • Outside power filters are among the most commonly used designs.

            BFA: Fair to good, depending on flow rate and media volume.

            MFA: Excellent.

            CFA: Good.  Varies with media volume.

 

            • Diatom filters produce very clean water very quickly, and are often moved from tank to tank to clear up the water in each one.

            BFA: None.

            MFA: Excellent.  Gets water very clean, but clogs quickly.

            CFA: Limited.  Can be used with activated carbon powder, but effective chemical filtration often requires more time than these filters are given.

 

            • Undergravel filters are often thought of as being synonym­ous with biological filters.  We have seen that other types of filters can perform this function; however, undergravel filters do it very well because they provide a very large surface area and a fairly slow water flow, giving the bacteria time to do their work.

            The filter medium is actually the gravel in the tank, which acts as a substrate for bacterial attachment.  If crushed coral or dolomite is used instead of gravel, chemical filtration can occur, but the options are limited.  Mechani­cal filtration ability is also limited, as sucking the dirt down into the gravel doesn’t really qualify as cleaning the tank.

            BFA: Excellent.  Simple and inexpensive.

            MFA: Poor.

            CFA: Limited.

 

            • Trickle filters, although not yet very common, are rapidly increasing in popularity due to their superb performance and advanced technology.  They tend to be rather bulky and expensive, and are usually used on larger tanks.

            BFA: Excellent.

            MFA: Fair to good, depending on flow rate.

            CFA: Excellent.