WOW - you have described my pond exactly, except I have 6 inch feeder fish instead of koi. Same exact size, round heater, warmer temps here, except our string algae is still brown that I can see. We've been in the 50's here the past week or so, and the fish have been hovering at the surface for days now.
I will be following this thread carefully to see if there is any way we can finally get rid of our string algae that was growing on every single thing in the pond that didn't swim. By the way, I have read that string algae does not die during cold winters.