Most people think that a garden pond filter works by getting rid of physical dirt from a pond leaving the water sparkling clean. A pond filter does strain a small amount of dirt but not much. What a pond filter does in real sense is act as a sewerage system that inhibits bacteria that are helpful to the pond and the life within it. It acts as a sewage system mainly because of the droppings from the fish and any excess food that gets deposited at the bottom of the pond and releases harmful toxins. A garden pond filter breaks down these harmful toxins into bacteria that can be helpful to the fish and plant life. However, an advanced garden pond pump filter strains more physical dirt but still not all of it. Before choosing a pond filter, the amount of fish in the pond has to be considered because waste will be proportional to the number of fishes.