There are many types of algae in a koi pond. It can grow in many forms but the most common are planktonic (pea soup) and filamentous (string-hair or blanketweed).
Over thirty years ago when I first became fascinated with the koi pond business, I serviced my pond accounts and expected the water to look as pristine as a swimming pool. Nowadays however, the primary goal is to achieve the proper balance. This approach both enhances water clarity and provides a natural enviornment for the fish to live. It cannot be overstated that managing algae plays the most important role in the effort to attain that balance.
In a pond, algae can actually be beneficial by keeping nitrite levels and other nutrients in check. In small amounts, algae can add oxygen to the system through the process of photosynthesis. Also, koi will eat algae in small amounts, especially when food has not been provided for them. This is why it is wise to occasionally hold off feeding your fish for a day or two. They will help keep algae in check.
Algae usually starts to show up in early spring (sometimes as early as February in Southern California) and will last through late summer. I have always been fascinated when it warms up in the spring, just how quickly algae can grow. It is just as fascinating to watch it die back as quickly in the fall.
When servicing my bi-weekly accounts in the spring, algae would literally take over a pond from one visit to the next. This occurs because the natual process of the beneficial bacteria takes a while to "wake up" to balance out the pond. Adding over the counter beneficial bacteria products such as Microb-Lift helps. But, many of these bacteria products will not be activated until the water temperature warms to over 60-65 degrees. In most cases, just waiting a few weeks until the water balances out works best.
During the day, algae produces oxygen. However, at night, algae consumes it. So, when you get an algae bloom, it is critical to provide adequete aeration 24 hours a day, especially at night.
I have found the best approach to managing algae and water clarity is a good filter matched with the proper pump. Besides removing unwanted waste and nutrients from the water, biological and mechanical filters also create an enviornment where beneficial bacteria thrives.
Additionally, ULTRA VIOLET LIGHT UNITS kill some diseased organisms and algae as the water passes over a high intensity lamp. This helps tremendously when trying to eliminate green water.
AQUATIC PLANTS - Aquatic plants such as lilies, shoreline marginals and floating plants consume nutrients for which the algae competes. They also discourage algae growth by providing shade that blocks the sunlight.
CHEMICAL TREATMENTS - Other means of eliminating algae are the use of algaecides. Although not my first choice, algaecides will provide an added "punch" when needed to get the upper hand when algae gets out of control. Be sure to follow all labels and reccommended dosage rates. Overdosing chemicals can cause a sudden die off of algae or plant life creating a "Ph crash". This will deplete the oxygen levels which will result in a fish kill.
SALT TREATMENTS - Adding salt to your pond in the correct dosage rates is a great alternative to chemicals. First of all, it is very effective when combating string algae, it is inexpensive to use, and it activates a protective "slime growth" on koi. It also kills certain parasites harmful to your fish. Digital salt test units are available for less than $100 online and can measure the exact amount of salt that you are applying.
Using a solution of no higher than .6% is acceptable. However, rates higher than .1% will begin to kill certain plant life. Never overdose and remember that salt stays in water and does not evaporate. So, it is meant to be a short term solution. Over a period of the next several weeks, either backwash or pump out the water and add fresh water a little at a time. This will eventually remove all of the salt. Use only pure salt in crystal form and with no additives. Either dilute it in bucket and pour it directly in or place it on a waterfall to dissolve slowly into the pond.
FLOCCULANTS - Flocculants are not algaecides and do not kill algae, per se. They work by gathering the suspended particles in the water to where they become larger. They are then trapped by the filter or drop out of suspension, thus clearing the water. A word of caution here. If too much flocculant is used, the gills of the koi will become clogged and they can suffocate. Again, follow all the directions on the label and it should be safe for the fish.
IN CONCLUSION, balancing algae in a pond is much easier to manage when taking control early in the season. Following the above suggestions along with taking a pro-active approach, will keep the pond life of both the koi and their owner just as balanced.