Koi excrete urine and produce faeces, and ammonia is excreted through the gill membranes. It is the job of the filtration system to remove waste which in the wild would be diluted by the large volume of water or washed away by moving water. Mechanical filtration .
Most filter media have a mechanical function.
Settlement chambers allow gravity to drag the solid waste out of the water by slowing the water flow. Such chambers usually come first in a filter. A vortex unit provides greater settlement, the water moves in a circular movement allowing solids to gather in the centre where they can be removed. In addition to baffle plates which slow the incoming water, brushes or matting can be used to strain the water. Biological filtration .