All ponds have different filtration needs. Some ponds need more biological filtration, others need more mechanical. Some ponds have zero filtration and remain clear all year. The pond size, water depth, amount of fish, sunlight exposure, trees, climate, and owner involvement all play a role in what kind and how much filtration is needed.
A skimmer effectively filters floating debris from your pond through a net. It also houses and protects your pump from large debris that could clog or damage it. Installing a skimmer to an existing pond can be challenging as it needs to be screwed on to the liner. However it is one of the most popular filters out there and many professional pond installers use it.
Biofalls are used not only for filtration but for creating cascading waterfalls and streams. Inside the black container there is media that serve as both mechanical and biological filtration. A biofalls paired with a skimmer is the industry standard for Aquascape Contractors.
Bead filters like the one above work by pumping water into a pressurized container filled with beads. These beads then grow beneficial bacteria that keep ammonia levels down and break down waste. They are expensive however they do provide strong filtration for ponds with lots of waste.
A canister filter is one of the easiest filters to install to an existing pond. A pump conected to the canister is placed inside the pond to filter water. No adjustments need to be made to the structure of the pond. However, like the bead, canister filters can be difficult to clean. Many do come with backwash mechanisms to periodically rinse the media but over time they will still need to be deep cleaned for maximum water clarity.