Some aquariums and some hang-on-the-back (HOB) water filters use cartridge filters for primary filtration. These filters fit into a special slot in the filter assembly to remove large debris; some are filled with activated carbon for chemical filtration. Many manufacturers sell pre-made cartridge filters and replacing them can be costly. Luckily, it is easy to make a replacement cartridge filter for many filter systems.
Remove the original cartridge filter from the aquarium. Most cartridge filters are mounted on a plastic skeleton. You can use this structure and replace the filter media.
Cut away the outside filter element. This is most typically some sort of bag, fine netting, or cloth. Its purpose is to remove large- and medium-sized particles from the water (such as bits of uneaten food). It also contains internal material, often carbon.
Pour out the internal material and the activated carbon. Activated carbon removes certain chemicals from the water, helping to clarify it. Open this over the trash; do not pour the carbon down the sink.
Replace the back of the filter media. What you need to do is recreate the bag that held the carbon (or other media). You can buy filter media at most aquarium supply stores or you can use cheesecloth that you can find at most grocery stores. If you use cheesecloth, layer it about four times.
Glue the material on with aquarium safe silicone. Depending on how your cartridge filter is made, you may be able to simply attach it with rubber bands. If not, a few drops of aquarium-safe silicone will work well. You can find this silicone at aquarium supply stores and some hardware stores. Let it cure according to the instructions before letting it come in contact with the aquarium water.
Lay the filter flat and fill with activated carbon. Even if there wasn't activated carbon in the original filter, it is a good idea to use it. You can purchase activated carbon in bulk online and at some aquarium stores. Fill the cartridge so it won't spill out. You don't need a lot of activated carbon in the filter. For a 55-gallon aquarium, 2 to 3 tablespoons is likely enough.
Glue the filter material to the front of the filter. Again, you may be able to use rubber bands depending on the design of the cartridge. Use aquarium-safe silicone for this as well. A thin coat will do.
Let the silicone cure according to the instructions on the tube. This is very important. When wet, the silicone is toxic to marine organisms, but safe when dry and cured. While you are waiting for it to cure, you can work on making other cartridge filters so you have extra.
Rinse the cartridge filter under running tap water. When activated carbon rubs against itself, the result of shipping or handling, some of the carbon turns to dust. If you place the filter in the water without rinsing, a black cloud of carbon dust will likely form in the water. Rinse until the water flows clear through the filter.
Add the filter back into the aquarium. It doesn't have to fit perfectly for it to work. As long as the majority of the water in the filtration system flows through it, the filter will work on removing debris from the system. Some filters have special slots or grooves where the cartridge filter fits. If you use rubber bands, it might not fit into these grooves. If it doesn't fit, you can lean the filter against the edges of the slots so water passes through it.
You can use a variety of filter media (cheesecloth, muslin bags, or cotton fibre) provided it is safe for the water. If you don't have the original cartridge filter, you can make one from acrylic or other food-safe plastics.
Do not use fine mesh materials like paper towels or coffee filters as filter media. They restrict too much flow of water and will cause the pump to run dry or the filter assembly to overflow.