How to solve water quality problem in my Koi pond?

This is a question I often get from my friends after they have seen my pond or heard that I am a Koi keeper.

Many pond owners have built their first pond without understanding the need for a proper pond filter system. Often, these are DYI ponds or ponds build by housing contractors with little experience in Koi pond construction.  They fail to understand the need for a proper water filter system, specially designed for Koi fishes. A Koi pond without a proper filter system would often results in pond water quality problem.  Symptoms would be murky or green water that looks like pea-soup, unhealthy or dying fish, foul smell and all these despite regular back-breaking pond maintenance tasks.

Although water additives would often help fix the symptoms, the long term solution would be to invest in a proper Koi filtration system. Best is to plan, design and build it right the first time. For cases where the pond is already constructed, I guess the most practical way to solve your pond water quality problem is to install an external pond filter system. This avoid costly and time consuming reconstruction of the pond and reduce risk of losing your fish during the process of upgrading your pond.

A good filter system would have mechanical filtration, biological filtration, AND  allows for ease of regular filter maintenance (like backwashing of filters, partial water changes). If your budget permits, choose one with UV steriliser system that helps to kill parasites and algae cells in the water (cause of green water). Some even employ a vortex, centrifugal system to draw solid particles to the center of the filter for more efficient mechanical filtration. These external filter systems come in various models and design to suit the capacity of your pond and your budget. 

Even with a proper filter system, do perform your regular partial water changes to remove the nitrates.