UltraViolet Clarifiers (UVC) help keep a pond free from unsightly green water than can prevent you from seeing your fish collection.  They work by shining ultraviolet light through the water passing through them. The Ultraviolet light kills the green water's cells and causes them to clump together. Now dead and clumped together it is easier for a filter to remove them from the water.  Generally with the right size UVC and filtration a pond can be guaranteed green water free.