Raw cotton will bio-degrade in water, so it's useless as filter medium.
John
Very true, cotton is organic so it is readily bio-degradable. The materials we use at my work are synthetic fibers, a plastic, petroleum based product. These don't bio-degrade easily, but they don't work well for mechanical filtration purposes because they are such a fine fiber and closed porosity. Unless you replace it constantly, it clogs up and shuts off the flow of water through it.
It would work fair as a pad or thin layer (as in a charcoal filter for a 10 gallon aquarium).
Gordy