Box Filters

 (This article was first published in January 1993.)

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If you’re like most people I know in the aquarium hobby, myself included, you probably started out keeping fish in a 10 gallon tank.  Many novices initially maintain hardy “beginner” fish, such as swordtails, guppies, and platies, which can live quite happily in a tank of this size.  Whatever kind of fish you kept and regardless of how you obtained the aquarium (purchase, gift, hand-me-down, etc.), chances are good that its filtration was provided by an air-driven box filter.  These filters are widely used in beginner’s tanks because they are inexpensive and easy to set up.

Just because they are often associated with such novice setups, however, and because they don’t cost a lot of money, doesn’t mean these filters aren’t any good.  On the contrary: they can do quite a good job if set up properly.  Proper setup and application are important because, despite their virtues, box filters do have certain limitations.

A box filter is actually a miniature canister filter which is driven by air flow instead of an electric water pump.  It shares with its larger brethren the ability to employ a wide variety of filtration media, thus giving it the versatility to provide biological filtration, chemical filtration, or both.  Like a canister filter, it must be located below water level, but because it is placed in the water, it cannot do its job as invisibly or unobtrusively as the canister filter.  On the other hand, it requires but a single, narrow air-line tube, avoiding the more complex hose arrangements of canister filters.

The operation of the box filter is simplicity itself (see diagram.)  Air is pumped to the bottom of a pipe which stands vertically in the center of the filter.  The pipe is raised a short distance above the bottom of the filter box to allow water to enter it from below.  As the air bubbles rise in the pipe, they pull water with them from the bottom of the box.  Water enters the top of the box surrounding the pipe and flows downward to replace the water that has been dragged out of the box by the air bubbles.  As this replacement water flows downward, it passes through the filter medium that has been packed around the ascending pipe.  (The medium is also held a short distance above the bottom of the box by a perforated plate, to allow water to flow freely out of the medium and into the ascending pipe.)

Because they are air-driven, box filters don’t have the high water flow rates of many motor-driven filters.  However, when properly set up, some box filters can attain the quite respectable flow rate of 100 gallons per hour.  Proper setup in this case involves an adequate air pump, an airstone at the air input to the filter, and a “chimney” that extends the ascending pipe several inches.  The small bubbles produced by an airstone provide greater lift than the larger bubbles that come out of an open-ended tube.  The “chimney” increases water flow by extending the time during which the rising bubbles actually pull water through the filter.

A high flow rate, although nice to have, is not essential for the proper operation of a box filter.  What is required is a continuous flow of oxygenated water over the filter media.  This will provide a beneficial environment for the nitrifying bacteria that colonize the surface of the media, oxidizing ammonia to nitrite and then to nitrate, a process known as biological filtration.

The list of potential filter media is long.  The “traditional” packing of a box filter is a layer of activated carbon on the bottom, covered by a layer of filter floss.  Since the floss is on top, the dirty water entering the filter is screened of solid particulate matter (mechanical filtration) before it flows through the carbon layer, which removes many organic chemicals, dissolved gases, dyes, etc. (chemical filtration).  Both the floss and the carbon have enormous surface areas exposed to the water, allowing substantial populations of nitrifying bacteria to colonize them.

Box filters are by no means limited to the use of these two media.  You may break with tradition and put almost anything you want into the box, depending on the effect you want to achieve.  Coral gravel or dolomite can be used to increase pH and hardness.  Peat moss will have the opposite effect.  Plain gravel, ceramic, or plastic media (sponge, floss, etc.) may be used to provide mechanical and biological filtration without otherwise affecting the chemical makeup of the water.  In short, anything you can put into a canister filter can also be used in a box filter.  The only limitations are that it must fit within the small box in sufficient quantity to do some good and it must not be packed so densely that air flow alone cannot draw water through it.

A box filter may be located anywhere in an aquarium, but it will operate most efficiently if placed near the bottom of the tank in an upright position.  This allows the rising bubbles to draw the maximum amount of water through the filter, as well as to circulate water from the bottom to the top of the tank.  This circulation improves aeration of the aquarium water, distributes filtered water throughout the tank, and draws unfiltered water toward the filter.

The mechanical filtration ability of a box filter depends on the rate of water flow through the filter and on the nature of the filter media.  As in any type of aquarium filter, the finer the mesh of the filter medium, the more effective its mechanical filtration ability.  At the same time, the more effective a particular medium is at mechanical filtration, the more easily it becomes clogged.  It is not easy to tell when  a box filter is clogged, as the actual water flow is invisible.  Bubbles continue to stream from a clogged filter, but they are no longer drawing enough water with them.  For this reason, it is a good idea to clean box filters at regular intervals.  (Of course, it’s a good idea to clean any type of filter at regular intervals!)  One visual cue that indicates when to clean a box filter is the appearance of the media: when it looks dirty, it is dirty.

When cleaning a box filter, it’s important to remember that too clean is worse than too dirty.   A dirty filter has loads of nitrifying bacteria clinging to it, doing their job.  Cleaning the filter removes or kills these bacteria; if they are the only source of biological filtration in the tank, the ammonia and nitrite concentrations are likely to rise and the fish will suffer.  If the box filter is the tank’s only filter, you should clean or replace only part of the media (say one-half or two-thirds) at a time.  If you have two box filters, or a box filter and another type, you should only clean one of them at a time.  By alternating your filter maintenance, you assure continuity of biological filtration, which makes for a more stable environment for your fish.

For maximum efficiency, a box filter should be run by an optimal rate of air flow.  (Too much or too little air will both reduce water flow.)  As mentioned earlier, airstones will usually provide greater lift, but they cannot be fit to all box filters, and the filters will work well even without them.  How can you decide on an optimal air flow rate?  Generally, the mixture of water and air exiting the top of the filter should appear to be about half water and half air.  If there are more bubbles than water visible, you should be able to increase water flow by bleeding off some air with a valve, or reducing your air pump’s output rate with a knob (if so equipped.)  If there’s more water than air in the mix (for example, if one bubble escapes the filter before the next one becomes visible), increase the air flow.  If that’s not possible, you need a bigger air pump.

Because of their limited rate of water flow, box filters are generally not suitable for large fish tanks.  Effective mechanical filtration requires a turnover rate of at least 4-5 tank volumes per hour.  A small box filter, properly set up, should pump about 45-50 gallons per hour, making it suitable as sole filtration for a tank of up to 10 gallons – which is another reason why they are so widely used in starter systems!  A large box filter can serve for a tank of up to 20-25 gallons.  For larger aquaria, several box filters may be used together, or box filters may be used in conjunction with other filter types to create total filtration that is adequate for the task.  For example, an outside power filter (which excels at mechanical filtration) can be used in a tank with a box filter loaded with coral gravel, which will provide chemical and biological filtration.

Don’t be misled by the simplicity, small size, or low cost of box filters!  Think of them as miniature canister filters and you will begin to understand their potential.  They are unmatched for versatile, cost-effective filtration.