There are several different types of filtration. From Mechanical Filtration: external mechanical pre-fitlers/bio media and skimmers, Biological Filtration: upflow gravity filter (filter falls), pressurized filters and UV Clarifiers / Ioniers to Natural Filtration: Bog Filtration and plant life in the pond.
Mechanical Filtration: Skimmers can be found here. Filter Media / Bio Media can be found here.
Biological Filtration: Filterfalls / Spillways can be found here. Pressurized Filters can be found here. UV Clarifers / Ionizer are found here