Mechanical filtration is far simpler to explain than biological filtration. It works to just to keep things spruced up a bit. We need to remove the particulate matter suspended in the water column before it decomposes and adds to the ammonia load. A sponge filter or canister filter with a filter cartridge will serve well to remove this particulate matter. If, however, you leave these filters in place without cleaning them often, you haven't removed anything, you've only moved it. So, if you use a mechanical filter, clean it often.
For my money, a hang-on-the-tank canister filter is the hot ticket. I usually hook mine up when I am doing water changes and other tank maintenance. Coincidentally, this is when everything gets stirred up the most and a bunch of detritus is suspended in the water column, just ripe for the pickin'. I run it for a few hours and then remove it. It also works very well for chemical filtration as will be discussed later in this text.
Protein skimming, also know as foam fractionation, does not fit solidly into the mechanical filtration category, but it fits better here than in the other filtration categories. Protein skimming is accomplished by injecting tremendous numbers of small bubbles into a contact tube filled with water. Since dissolved proteins tend to stick to the air-water interface of the bubbles, this causes protein foam to be generated, which is collected into a waste cup thereby removing the dissolved proteins from the tank.
Protein skimming is not mandatory, but it is very, very popular. It helps to maintain exceptional water quality, and should be considered an essential part of a well set up system.
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