How Much UV Power Will My Pond Need?

As a general guide, an ornamental pond (without any fish) will require no less than 1 watt of UV power per 1000 Litres of water.  A lightly stocked fish pond (i.e. small hardy breeds such as goldfish) will require around 2 watts per 1000 Litres.  A Koi stocked or heavily stocked pond will require at least 4 watts per 1000 Litres.

The reason for scaling up the ultraviolet radiation for fish and Koi ponds is these inhabitants produce more waste, which usually leads to more aggressive algae growth.  It is far better to use a UV Clarifier which is over-powerful than under-powered, as an under-powered UVC will not be able to kill the algae faster than it is replicating, so suspended algae will win the war and green water will be prevalent.