I am in the same boat, I have a blanket algae problem that I thought would clear up a bit during the winter. So far it has not. I did lose one fish the other day after a few nights of 17 degree temperatures. I couldnt find anything wrong visually with the koi and water tested fine with the exception of the PH being a bit high. I am curious to hear any algae reducing ideas as well as once spring hits my pond will be overwhelmed I think with the string algae or blanket weed. Anyone know if the cold is what might of killed my koi? I am at a loss as why he died. All other koi seem fine. Could it be the consumption of the algae itself?