Homemade Box Filters

 By Lawrence L. Feltz


Years ago I began making "box filters" or "corner filters" using a system Larry Konig presented in his publication Portrait Of The Guppy (Elizabeth, NJ: E.G. Publishing Co., 1967, pp. 74-75)

He says, "For my use, I invert a plastic funnel in a small dish, fill up the space with marbles about half way up and then cover with a layer of coarse gravel. The funnels...have a piece of stiff plastic tubing added to heighten the stem for better results....The airline can be placed under the funnel ... The plastic funnels should have four  notches cut out of the edge around the wide mouth of the  funnel that lies flat in the dish or under the gravel, to insure good suction action. The size of the notches should be one quarter inch wide on the edge to a triangle point one fourth or three eighths inch long. ".


I first made this kind of filter in the 1970's using margarine tubs and store-bought plastic funnels. Instead of the marble/gravel mix I used only aquarium gravel. The result was a small (portable) undergravel filter. It worked very effectively. I experimented with a wad of filter floss in the cavity made by the funnel mouth. While this worked well, it got very dirty. It was a mess to clean. I abandoned that idea.


I also experimented with the idea of drilling the margarine tub with many holes to increase circulation and efficiency. This appeared to work well. I discovered the design flaw when I went to move the filter. When removing the filter for cleaning the trapped dirt fell out through the drill holes and back into the aquarium before I could remove it. My later designs used a solid bottom. They seem to be as effective as "store bought" box filters.


 When life circumstances forced a hiatus in my fish keeping, I put a few of these filters into storage. In 2001, when I decided to get back into the hobby, I found them again. They were old and discolored. I did not have enough for all the tanks I was planning for. I figured I needed to replace them.


Originally I thought I would just buy some filters at the local pet store. To  my amazement the hobby appeared to have changed tremendously in the 20years since I had kept fish. I could not find box/corner filters in the pet store. The manager of the local aquarium said, "No one does it that way". Every one uses power filters. He spoke of "power heads", "skimmers", "wet-filters", "wet/dry filters", all things with which I was completely unfamiliar. He recommended an individual outside hang-on motorized filter for each tank. I was hoping to resurrect 25 old Metaframe tanks. It was apparent that would cost me a fortune!


I had one beat-up old air pump (a Silent Giant) that I pulled out of storage and put into service using the old margarine tubs/funnels that I cleaned up and sanitized. They were suddenly looking pretty good to me. I began to think of saving margarine tubs, and buying new funnels. But we don't eat margarine any more. And I hated the idea of paying the price the local Walmart was asking for funnels. Then I hit upon the idea of substituting something else for the funnel. That made me think of a reprint from The Guppy Roundtable where the author had made a filter from bleach bottles. It that case, the neck of the bleach bottle provided the funnel-shape. But the bleach bottle would be too big for a 5 gallon aquarium.


Due to the plastic revolution there are a multitude of smaller sized bottles readily available in our modern society. A quick trip to the neighborhood plastic recycle container yielded a treasure trove of different sizes, shapes, and colors. I have experimented with many different combinations.



My favorite combination for a 10 gallon aquarium is made from the round, flat-bottom (actually slightly concave), clear-plastic container that Gatorade, applejuice, and a host of other things come in (the 64 and the 48 oz sizes both have the same diameter - about 4.25 inches). I cut around the plastic 3 - 3 1/2 inches up from the bottom of that kind of container for outside of my homemade filter.



In combination, I prefer the ubiquitous bottled-water container (small sports drink or soda bottle) for the inside -- this provides the funnel shape (12 and 16 oz sizes have a diameter of 2.75 inches). I use the 3 - 3 1/2 inches from the top (the tapered part) plus about 3/4 inches more. The extra 3/4 inch I fringe with slits about 1 inch apart that I then bend out (to make a flange) at a 90 degree angle. When I put the funnel shape flat inside the larger container, the tabs (when covered with gravel) help hold the funnel in place.



Next, I cut a "V" shape (two sides of a triangle shaped opening - leave the third side uncut to serve as a hinge) near the base, but on the slope of the funnel (keep the cuts small -  1/4 inch). Through this "hinged-triangular opening" I route 3-4 inches of flexible 3/16 inch airline tubing. (You may prefer to choose a length of tubing long enough to directly reach the air valve attached to your air source. Be aware, however, that when it comes time to move or clean the filter, the extra length of tubing can create a mobility problem.)



 I recommend using the aforementioned 3-4 inches of the flexible 3/16 inch airline as a connector.  This tiny piece of tubing is the "lynch pin" to the tubing you drop into the tank from your air source. At the outside end of this small piece of flexible 3/16 inch airline tubing, I insert a small portion (1/2 inch) of rigid 3/16 inch tubing (about half its length -1/4 inch). Next, I attach the rigid 3/16 inch tubing to a length of tubing long enough to reach the air valve attached to my air source. This small link makes future relocating and cleaning much easier.



One last thing! To restrict the air flow and cause efficient water exchange, I reinstall the cover of whatever container I am using for the inner funnel. First, drill a hole in it. I make mine 3/16 inch in diameter. Again, from Larry Konig, The funnels...have a piece of stiff plastic tubing added to heighten the stem for better results... I formerly inserted a piece of tubing into the drilled opening (3/16 rigid) to heighten the stem. After experimentation I decided this was extra work and expense that did not improve the filter. Recommendation: just go with the drilled hole in the top.


Place the funnel flat (tabs extended) in the bottom portion of the filter. Fill the space between the inner funnel shape and the round outer tub with gravel.



 The difference in diameter between inner and outer container provides about 1 to  1- 1/2 inches space for your chosen filter medium. For years I used #3 quartz aquarium gravel, the size which seems to be most common at local aquarium stores, with good success. Recently, I have begun using small stones. I love gathering mine from the ocean's edge.



In the interest of  sanitation I use bleach (ratio 1:10) to soak (30 minutes) and clean all the found materials before putting them into service for the first time (rinse with plenty of hot water). For maintenance, I remove the "dirty" filter media. Put it into a strainer/colander and rinse with aged tap-water or used aquarium water (to preserve the "beneficial bacteria"). Return it to the homemade "box-filter" and place it back into service.


Again, in deference to Larry Konig, The funnels...have a piece of stiff plastic tubing added to heighten the stem for better results. I have experimented with cutting the water bottles in such a way as to make a taller funnel (to heighten the stem for better results). I don't think it does improve the performance. And, bigger is not more beautiful in the aquarium. But you should experiment on your own. Let me know of any "improvements" that occur to you.


A final note, as the result of necessity and in the interest of experimentation, I have tried a multitude of the smaller sized bottles available in our modern society - in different combinations. I have used, with success, the upper "funnel" portion of the 48 and 64 ounce "Gatorade" bottles as my funnel ( a bit weird looking, the wide mouth makes for a broader mouthed funnel). I have also had success, using the upper "funnel" portion of 2-liter soda bottles.


I also experimented with various sizes and shapes for the bottom (outer container). Here too, I used the 2-liter soda bottle. It works. But, its unique bottom (not flat-bottom) is a problem for me in that the funnel does not sit flat. Furthermore the diameter was smaller, and the combination less space and the not flat bottom restricted the amount of "filter media" it would contain. I also find them to be "tippy" if bumped accidentally during normal tank maintenance.


In my smaller "breeder tanks" I use a miniaturized version of the filter described above. I have successfully used the bottom of a bottled-water container as the base. For the top "funnel" shape, I use the upper portion of a bottled-water container. It is a very tight fit and does not allow for much filter media, but it does a good job of both filtration and aeration.



The combination I like best is the flat bottomed 1/2 pint container I get from the deli as the base and the upper portion of a bottled-water container. This combination results in a stable, broad base (allowing for maximum filter media). It has a lower profile and is not "tippy". While it takes a bit more tank space, I find it functions very well.