There are as many filters as there are pumps in the water garden industry. 3 basic needs are present when trying to filter your water garden:
1. A pump is required for any filter so you must protect the inlet of your pump from plugging up, this is sort of pre filtering of large debris. It it usually involves a screen of some sort on the intake of your pump. For some small (5' x 8' x 2' deep) ponds a bottom filter is about all that is required for filtration. This filter connects to a pump in the bottom of your pond.
2. Skimmers are probably the only real innovation in the water garden industry to date. All skimmer boxes today are made of plastic, a concern here in the freezing north country, and in hot climates where plastic can take on its own shape in the mid day sun. We believe that most water gardens benefit from skimming as this is where most debris first enters the pond ecosystem.
Today's skimmer boxes can accommodate a submersible pump or a plumbed suction line from an external pump. These units go in the ground behind your liner outside of the pond. They have a leaf basket and a filter matt which keeps the pump clean and free of debris.
3. Water is moved from the pump to some sort of filter container or vessel. These can be complex systems of media that must be cleaned periodically, or as simple as what many refer to as a bog, or bio filter. These filters from size; the larger, the better to some extent. We find bog filters to be very easy to care for. They require only a few hours per year to maintain, and they are essentially plants filters and mature into very pretty gardens in and of themselves.
Most "kits" sold today come with a waterfall filter. This has its use in very limited spaces, but not in an easy care garden. They must be cleaned frequently and are also vulnerable to freezing and hot temperature movement.