Calculating the size of the biological chamber of a trickle filter is not a complicated task. We will use an example to illustrate this point. Substitute your own numbers and recalculate the results if you are planning on building your own filter, or if you would like to calculate the exact content of a filter you see in a magazine or store.

The premise is the following: for any rectangular or square box that houses a medium for biological filtration, the gallon content can easily and quickly be calculated in two fashions:

• take the measurements in inches and multiply length by height by width.

Divide that number by 231, and the result is the gallon content of that container.

• Or, take the measurement in feet, multiply length by width by height, and then by 7.5. This particular method comes in handy, for example, when calculating the content of an aquarium.

A box that is 14 x 18 x 16 can hold 4032: 231 = 17.45 gallons of media. A filter that has a chamber that is 12 x 4 x 15, can hold 3.11 gallons of media. An aquarium that is 5 feet by 18 inches by 24 inches, holds 5 x 1.5 x 2 x 7.5 = 112.5 gallons of water.

Underneath an aquarium two of a filter's dimensions are usually determined, (for the most part), by the space available inside the cabinet: the width and the height. The third dimension, the length, is usually more flexible. To calculate the space needed use the two constraining measurements to calculate the third. For example: your cabinet is 5 feet long, by 20 inches wide, by 26 inches tall. The inside dimensions of the cabinet, meaning space available to place a trickle filter, are 48" long, by 16" wide, by 24" high.

• Your aquarium is a 100 gallon tank. You are planning to install a filter capable of handling a heavy load of fish and invertebrates. You are building your own filter and need to determine the dimensions.
• Based on the recommendations made with regard to surface area required for biological filtration, we first need to determine how much of such surface we need for this aquarium. Using the numbers suggested earlier:

100 x 2.25 (average) = 225.00 square feet. You have decided to use small Tri-Packs as the medium. These small round plastic media have a surface area of 14.7 square feet per gallon equivalent. 225 divided by 14.7 or 115.3 gallons of Tri-Packs. This would require a rather large filter (15 gallon capacity).

• Because you do not want to build such a large filter, you look around some more, and decide to use some other media, in this case Bio Blocks from World Class, with a surface area of approximately 20 square feet per gallon equivalent. 225 divided by 20 = 11.25 gallons needed.
• This seems acceptable size and cost-wise to you, and you now set out to calculate the dimensions of the filter's biological box.
• Given the cabinet's constraints in terms of width and height, you know that you cannot use more than 16 inches in width, so you settle for 15. Height-wise you have 24, but you need to leave some space for fittings on the top of the filter, so you settle for 21 total height.
• Because this total height also includes the sump space underneath the biological box, you must first deduct that height from the 21 inches. 5 inches is a good height for the water level in the sump. 21 - 5 leaves 16 inches.
• What we have determined so far, is that the width of the box will be 16, and that the height of the biological box will be 16 as well.
• From these numbers we must deduct the thickness of the material used (1/4 inch in our case), times 2 (once for each side), as the 16 inches is an outside dimension, and to calculate volume, we need inside dimensions.
• This leaves 15.5 inches for the width. The height is only affected on one side (the top) because the bottom is open and in the sump. The two dimensions we thus have are 15.5 and 15.75.
• The following equation represents the third dimension (Y) that we are looking for:

15.5 x 15.75 x Y = 11.25 x 231

• Let me explain. Earlier we determined that volume is equal to length times width times height (in our case 15.5 times 15.75 times Y the as yet unknown length). We also saw that to go from volume to gallons we divide by 231. This means that 11.25 times 231 = the volume = length times height times width.
• Transforming the above equation gives us:

Y = (11.25 x 231) divided by (15.5 x 15.75) or Y = 2598.75 divided by 244.125 = 10.645 inches long for the box.

• To verify that our box indeed holds the number of gallons of media needed, all we now do, as a double checking procedure, is to multiply length by width by height: 10.645 x 15.5 x 15.75 and divide that number by 231. Doing so generates: 2598.71 divided by 231, or 11.2498 (111.25) gallons.
• The dimensions of the biological box are: Width 15.5" plus 2 times 0.25" or 16 inches, height 15.75" plus 0.25" for the top plate, or 16 inches, and 10.65 plus 2 times 0.25 or 11.15 inches.
• This biological filtering box now needs to be fitted in an appropriate sump. The sump needs to be 16.5 inches wide. 10 to 12 inches is a good height for a sump. The total length of the sump (filter) is a matter of personal decision and preference.

Usually you will want enough space in the front part of the filter, to be able to place additional compartments in the sump, and maybe even a protein skimmer. A sump that is 16 inches long (plus 16.65 from the biological chamber, in our case anyway) will allow you to do so, but you can select any number of inches you feel comfortable with. In our case the sump would be around 32.5 inches long. The only constraint on the total length of the filter, is that you have to leave enough space available underneath the cabinet for your pump(s), and valves, and any other equipment that must be installed there. This is most important.

Whatever the size of your tank, you can use the above example to calculate the size of the filter you will need. All you need to do is substitute your numbers for the ones we used. Does it pay to build your own filter? It greatly depends on how handy you are, and whether or not you can work with plastics and know the bonding techniques and tricks. You can order the cut pieces of plastic easily from a local dealer and assemble them in your house, or garage, as long as it is well vented, not too cold, and not humid at the time you will be bonding.

Always use high strength bonding agents. Let the filter dry for at least several hours, better still, let it sit overnight before putting any water in it. Do not try to bond in high humidity areas, or in areas where the temperature is lower than 70 degrees. Before you bond pieces together, make sure that they all fit.

Use masking tape, or something similar, to put the filter together to double-check that all pieces are of the proper size. Once glued you cannot get them apart. All edges need to be checked. They should be smooth, especially if you are using liquid bonding agents that dry rather quickly. No gaps are allowed, the pieces must fit tightly all around. Gaps cannot be filled with liquid bonding agents. To do so you must use a paste-like acrylic cement. They are available from companies such as Weldon, Cadillac Plastics, and C.R. Laurence. They need much longer to set than other types of bonding agents. If you have never glued plastics such as acrylic before, you must try first on some pieces of scrap.

The filter will require several holes for water inlets and outlets, and also for air inlets. Drilling acrylic is not difficult. Again, practice on some pieces of scrap first, this will give you the feel for it. Use special acrylic drill bits if you can. They make for a smoother hole. Although carbon drill bits are probably the best bits around, they are difficult to use on acrylics if you are not used to working with them, especially in hand-held drills. Using a drill press is recommended for drilling holes, but hand-held units will work too, providing you practice a little first.

An article on how to build trickle filters was published in the January 1988 issue of FAMA (Freshwater and Marine Aquarium). If you are planning on doing so you may wish to review that article.

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