Pond filters (when placed BEFORE the pump) protect the pump from clogging from debris in the pond. More importantly, they purify the water and the environment, removing debris and excess nutrients from the water using 2 simple processes:
Mechanical Filtration acts as a barrier, trapping dirt and debris (usually in an easily-cleaned foam block) so that it can be removed. It is the first defense for clean, clear water.
Biological filtration is 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.
In the process the water is made clean. The proper balance of bacteria against waste is the most important part of keeping a pond in healthy condition for fish and plants. Too little bacteria or too much waste will turn everything to muck. Read More HERE