Pond filters protect the pump from clogging from debris in the pond. More importantly, they purify the water and the environment using 2 simple processes:
Acts as a physical barrier, trapping suspended dirt in a foam block so that it can be easily removed.
--courtesy Hagen Pets
Performed by beneficial bacteria which grow naturally in the filter media. These microscopic organisms convert the harmful toxins (ammonia and nitrite) which are manufactured by fish and decay processes into much safer nitrates, which are an excellent plant food.
There are three types of useful pond filters:
Submersed Filters are easy to hide, although they take up valuable space inside the pond. They are usually attached to the intake side of the hose to trap particulates before they reach the pump, thus protecting the pump as well as filtering the water. They come in different sizes and are usually fairly easy to clean although the pond keeper will have to wade into the pond or in some other way retrieve the filter for cleaning, and waste may spill back into the pond if not handled correctly.
In-line Filters are usually self-contained canisters which are sealed and operated under pressure. They can be used submerged or above ground.
External Filters are the largest and best filters. They are located above ground (or partially buried) somewhere near the pond. Water enters the filter from the pump and trickles through several types of media to clean and detoxify the water.
Although they need the least cleaning and provide the most ease of cleaning, the pump is not protected from floating debris because the pump is placed before the filter. To protect the pump, it is best to attach a submerged filter on the intake side of the pump and an external filter on the output side.