• The science

How does a pond filter work?

Biological filtration performs a process known as the nitrogen cycle. In this cycle, organic pollutants that enter the pond via fish waste, decomposed plant matter or excess fish food are converted from toxic to non-toxic substances using oxygen and bacteria.

If the harmful substances are not broken down by oxygen and bacteria then algae will grow and fish death can occur. It’s the same process that occurs in aquariums, just on a larger scale.