The basic concept here is to remove as much solid feces and uneaten food as possible before it degrades and fouls the water with nitrogen in the form of ammonia. This will reduce the ammonia and nitrate load to the aquarium by roughly 10% to 20%. The key identifying property with mechanical filters in the aquarium is that the filter will need to be cleaned or replaced weekly. If it doesn’t plug up weekly it’s probably not really acting as a mechanical filter.
Types of Mechanical Filtration
There are many types of mechanical filtration, typically dependent on the type of filter they are in.
By far the most common cleaned mechanical filter is the fuzzy polyester fabric cartridges filled with media in cartridges on hang-on-the-back filters. These materials have pores sizes of about 1,000-micron, one millimeter or 0.040 inch. The cartridges are easy to remove and clean as frequently as one desires. The filter manufacturers recommend you replace the HOB cartridges on a frequent basis, such as once a month. Ignore them, they just want your money.
Mechanical filtration in canister filters is typically done with polyester fiber floss (i.e. Polyfil). This floss stops fish feces and uneaten food in a brown viscous film on its surface. Like all mechanical filters this floss needs to be cleaned or replaced weekly to be effective.
Still another purely mechanical filter is bonded polyester pads (one type is a “Pinkie” pad). These polyester pads are useful for purely mechanical filtration in canisters.
A 40 ppi foam as the first stage in a canister filter DOES act as a mechanical filter and needs to be cleaned regularly, like once a week. A 20 or 30 ppi (pores per inch) foam/sponge in a canister filter is NOT a mechanical filtration media. A 20 to 30 ppi foam is a biofiltration media.
A sponge filter of 40 ppi foam is a very weak mechanical filter. There isn’t sufficient flow through the sponge to do a very good job of mechanical filtration. One should just clean the outside of these filters on a regular basis, but no great harm will come from not cleaning them.
For high end cleaned mechanical filtration the best mechanical filter is a bag arrangement in a sump, with a 100-micron bag filter or filters. They are easy to clean and last a very long time. Joey (the King of DIY aquariums) has gone over to bags in his sump designs. Many use polyester pads like the pinkie pad in their sumps quite successfully.
One misunderstood type of mechanical filtration is a settling aquarium filter. There are a host of circular flow settling filter designs on YouTube. This type of filter needs a large container, like at least a five gallon bucket. The flow is in and out very high in the bucket, allowing large particles like feces to settle to the bottom of the bucket. This bottom is then occasionally drained to eliminate the brown settled “gunk” from the aquarium system.
If this “gunk” is drained once a week from the settling filter or so it works as a mechanical filter. But if this settling filter is only cleaned every six months things change. Over the span of about one month the “gunk” will convert from “bad gunk” to “good gunk”. The brown gunk will become a floating mass of biofloc with huge numbers of beneficial bacteria.
When such a settling filter is drained with removal of the brown biofloc there is a real chance fish will be impacted to some degree by ammonia poisoning. Note that some put very puffed up filter floss or pillow stuffing in such a settling filter where it stabilizes the floc and acts as a very good biofilter.
Uncleaned Mechanical Filtration
Click on this link to better understand an alternative to cleaned mechanical filtration:
6.1.2. Uncleaned Mechanical Filtration