Mechnical filtration is incorporated into most filters so that solids such as uneaten food, dead plants, solid fish waste and suspended particles can be effectively removed from the aquarium.
Without mechanical filtration, biological filtration would need the solids to rot and break down, resulting in extra load on the biological filter.
Mechanical filteration usually acts as the first stage of filtration in a filter. This is why sometimes it is also called a pre-filter.
Mechnical filter is essential because it:
Mechanical filter media includes
They all come in different shapes and sizes. If you want to save cost, you can buy them in bulk from your supplier because they don't have an expiry date and it is just a matter of time before you need them. In fact, it might even save you some transport costs of buying them in several trips each time they run out. If you have access to a bigger store with a wider variety and range of products, you may even be able to find them in large sheets. You can then cut them to size to suit your filter.
Caution: What you SHOULD NOT do is to find your own substitude materials. Some sponges used in matteresses or pillows or toys have anti-mold, anti-fungus or anti-bacteria chemicals on them that are harmful to your biological filtration and your fish!
Depending on the quality of the material, you may be able to wash and reuse mechanical filters a few times before they deteriote too much and need to be replaced. Filter sponges and scrubblers can be washed and reused many times but filter wool tend to break down fast and have to be replaced. If you use filter paper in your system, replace and throw away the old piece when it is full. Don't wash them because it is impossible to get all the super fine particles out. You would only loosen some of them and let it go back to your water if you do!
Mechanical filters should always be arranged so that the water passing through meets the coarser filters first before the finer filters so that the filtration is more efficient. (eg. filter paper should always be at the last stage of mechnical filteration before the water goes to bio filter)
The illustration below shows why you need to have finer materials after the coarser materials.
As you can see, if you put the course filter above the finer filter, the waste load is distributed evenly. As a result, you would need to change the filter less frequently because different filters are filtering different sized particles.
If you arrange it such that the finest filter is above, all the waste is trapped by the first layer. As a result, it becomes "saturated" fast and you will need very frequent maintainance. If the filter with larger pores are below the fine filter, whatever particles that could squeeze through the fine filter will not be trapped by the coarser filters below it.