How Do I Prevent My Pond From Going Green?

The best way is to install an Ultra Violet Sterilizer (UV Clarifier)
that has enough wattage for the size of your fish pond.

An 8 watt UV light is usually adequate for a fish pond up to 5,000
litres.

Water turns green or brown usually as a result of the build up of
microscopic algae cells (4 microns across) in the pond.

This is further increased when there is a lot of sunlight. The
algae feed on increased nutrients in the pond, present during the
warmer summer months.

Due to the microscopic size these single celled algae can not be
removed from the pond by a pond filter at this stage.

When pond water containing algae is pumped through an ultra violet
clarifier it is subjected to UV light which breaks down the cell
wall, killing the algae.

This causes the algae to clump together. As large amounts of algae
clump together they form a much larger mass which is easily removed
by your pond filter.

Source: Garden Pond Filters And Essential Water Gardening Products