Posted: 10/02/2015 at 13:54
blanket weed grows by using nutrients in the water, the koi are producing the nutrients (i.e. fish poo) as they get bigger they produce more poo - it could be why its only just started being a problem.
You can use barley straw, a UV filter addition, chemicals (not recommended) or shade part of the pond either with natural plants (water lily's) or with shade cloth in summer. or a combination.
I use barley straw, water lily's covering nearly half the surface and a UV filter. one of my friends has a filter system that runs thru a bed of watercress and then water reeds to take the excess nutrients out. works really well!