Biological (External Box) Pond Filters
External Biological box filters, as the name suggests, sit outside the pond & process the water that is pumped to them from a pond pump submersed in the pond water. Through a process of mechanical filtration particulate material is trapped & can then be removed. Biological filtration utilises aerobic bacteria to break down the most toxic products of fish waste & organic decay & together the two processes clean & purify the pond water. A filter is a useful component for most ponds & absolutely essential for ponds containing fish.
Box Filters must be situated above the maximum water level.
See also "Pressure Filters", "Submersible (in-pond) Filters" and "Floating Filters".