Although the algae that create green water are too small to be filtered out, all is not lost since the advent of ultraviolet (UV) light treatment and ultraviolet clarifier (UVC) units.

UV units are available as separate items or integral to a filter system, such as those from Pondkeeper but however they're supplied, the principle is the same. What you'll find is a tubular UV lamp sealed in a clear protective tube. Pond water is directed over and around the UV lamp at the recommended flow rate so that the UV light can disrupt the algal cells enough for them to clump together. These clumps can then be filtered out.