There are two main types of pond filtration. mechanical filtration witch removes the larger debris, while a Biological Filter looks after the water quality to keep the fish healthy and happy. Most modern filters combine these methods into one unit for convenience and ease of installation.
beginners believe a pond filter works by removing physical dirt and it is this which keeps the water sparkling clean.
In fact this is not the case, although a small amount of suspended particles are strained out by the biological filter.
A biological filter works like a mini sewage system for the pond. In simple terms the media within the filter creates a huge surface area, which becomes home to many millions of helpful bacteria. It is vital to keep these oxygenated by passing pond water through the filter 24 hours a day.