Green pond water is one of the most common types of problems for a backyard pond. This "greening" of the water is caused by a single celled algae. This algae grows because there is a nutrient imbalance in the water causing nitrate and phosphate levels to become elevated. There are several reasons why this occurs. One very common reason is due to muck and sludge building up on the pond floor. This layer of muck is composed of leaves fish waste and other waste that settles on the bottom. When this muck decomposes it releases nutrients in the water which then in turn provides food for the algae. This cycle is what turns a crystal clear pond into a swampy mess.
There are many other reasons for algae build up to occur. Another popular reason is having too many fish in the pond. With more fish comes more waste from the fish which turns into nitrates. These nitrates only increase the algae. This situation can occur more frequent and rapidly when there are numerous large fish in a smaller pond. Koi fish are relatively small at first but can grow very large and produce a decent amount of waste.
Another reason for algae filled water is due to a lack of pond plants. These pond plants are natures way of naturally filtering out the algae. The pond plants consume the nitrates and phosphate which starves out the algae. Pond plants are low maintence and easy to install.