Floating aquatic plants are usually selected as the ideal species of vegetation and these involve mainly reeds, duckweeds and water hyacinth as they have bottom root networks that can capture the waste more efficiently than the other plant species. Use and implementation of this concept is similar to the construction of wetland systems in treatment of natural wastewater and hence, application in koi ponds is very much similar in working concepts except that it is considered a scaled-down version of the whole process. Setup of the system is very much straightforward as what you need is a separate compartment being built close to your pond and the water is diverted to it after it has undergone primary solid waste removal, usually accomplished using a vortex chamber filter. Both are usually used in conjunction to support each other and the main purpose for doing this is that the water before being fed to the vegetable system has to be free without presence of large particles so that clogging problems will not occur and the natural filter will take care of the rest. Ideally, plant-based filter system overall layout should constitute and made up of at least 25% of total koi pond surface area in order to be considered an efficient setup.
Vegetable filters have several advantages over normal conventional mechanical and chemical setups. First of all, use of plants is generally considered far cheaper. Apart from that, it is also considered environmental friendly as it will void the use of huge amount of electricity to support high powered pumps needed in other complex setups especially the need to drive the water through compact filter media. Head loss and pressure drop caused by blockage is often a prevailing problem associated with conventional pond filter system. Instead, by using a plant-based process, normally a typical low wattage pump capable of transferring the water to another elevation is more than sufficient to move things around and get the system running. Another reason worth mentioning here is that growth of natural vegetation actually attracts and promotes propagation of microorganisms and underwater aquatic species which in turns provides extra natural foods for your koi fish. If well-maintained, the system is actually self-sustaining without needing much intervention.
The only problem that lies with the whole setup is often with the time that has to be spent on pruning and clearing the extra growth of plants so that new younger vegetation can establish and new growth will consume the nutrients much faster. Apart of that, one of the disadvantages is that the water can sometimes lead to sudden increase in mosquito population which finds the chance to breed and grow within the root networks. However, this can be easily overcome by using mosquito fish or if you prefer other aquatic species, platy and guppy can be introduced as well to the vegetable filter compartment. Related: simple guide on how to construct koi pond