Controlling algae in your Koi pond.
Biological filtration turns ammonia into nitrate which is harmless to fish (unless at extremely high levels) but the disadvantage of this is that algae love nitrate and you get an algae bloom.

There are two types of algae problems, green water and blanket weed. Green water is caused by microscopic algae in the water, it is not harmful to Koi, actually it is beneficial, the Koi eat the algae and it enhances their color, but you can't see them!

Also in summer the algae use oxygen and leave the fish gasping. There are various ways to get rid of the algae : a vegetable filter, plants will use the nitrate so it is not available for the algae; an ultra violet (UV) filter kills the algae as it passes through; algaecide chemicals can be used but the problem will just recur; magnets placed on the filter pipe will disrupt algae cells internally, killing them or preventing them reproducing.

Blanket weed is filaments algae and forms long green strands, it is not really a problem, it uses up nitrate and stops green water occurring, but it is unsightly. Vegetable filtration, algaecides and magnets will all work on blanket weed, but UV filtration will not as the algae has to pass through the filter to be killed and blanket weed is attached to the pond walls.