Mechanical filtration
Removes solid waste before it breaks down. Examples include settlement chambers, sieve filters, brushes, filter mats, drum filters, and prefilters. Mechanical filtration should be easy to clean.
Biological filtration
Provides surface area for bacteria that process fish waste. Moving bed filters, shower filters, trickle towers, mats, ceramic media, and bead filters can all support biofiltration when sized and maintained properly.
Gravity-fed systems
Often used with bottom drains and larger koi ponds. Water flows from pond to filter by gravity, then returns by pump. They are efficient but require planning during construction.
Pressure filters
Common in smaller garden ponds. They are compact and easy to install, but can be undersized for heavy koi loads and may need frequent cleaning.
Shower and trickle filters
Expose water to air while passing over media. They can support strong biological filtration and gas exchange, but may be noisy and visible.
Bog or plant filters
Use gravel and plants to polish water and absorb nutrients. They can be useful in water gardens, but they should not be the only life-support system for a heavily stocked koi pond.
UV clarifier
Helps control green water algae by exposing water to UV light. It improves clarity, but it is not a substitute for mechanical and biological filtration.
Aeration
Air pumps, diffusers, waterfalls, and moving water help oxygenate the pond. Aeration becomes critical in warm weather, during medication, and under heavy feeding.