New ponds are especially prone to algae blooms until the whole ecological system of the pond matures, which may take several years. Ultraviolet lights are designed to kill any algae floating in the water. Algae blooms in ponds can turn a perfectly clear pond into a thick pea soup looking pond in a short period of time. The way ultraviolet lights work is when the algae, in the water, passes through the unit it is exposed to a very bright ultraviolet light. This energy from the light destroys the DNA in the algae which in turn causes it to eventually die.
As the algae cells die they will begin to stick together so that they can be filtered out of the water. Selecting the proper size light is very important. If the light is not big enough for the pond you will notice almost no effect. Additionally, if the water flow through the light is too fast, it will also reduce the efficiency of the light to where it has little effect. Ultraviolet lights are not effective against string type algae because it attaches to the pond and does not flow through the light.