One of the least understood and naturally occurring biological elements in a Koi pond is algae. There are two types of common algae. One is planktonic algae, (green water) and the other is string algae, (long strings of green algae) that cling to rocks, waterfalls and pond surfaces. Planktonic algae are easily controlled with the use of an ultraviolet sterilizer. String algae can also be controlled by the use of either bacteria or chemicals. Understanding the organic balance of your pond, along with good mechanical and biological filtration, is the key to a crystal clear pond!