Chelated copper will kill everything in the pond and all the frogs that live around the pond. I would just ignore the algae until next winter when the pond freezes over. Then get 15 or 20 bags of barn lime from Tractor Supply and spread it across the ice surface. In the spring it will melt and fall to the bottom of the pond raising the PH level and the pond will once again be usable. If you have a lot of undergrowth in the bottom of the pond that you need to kill off, then the chelated copper will take care of that. Spreading chemicals on the ice surface is the safest way to get treatment to the center of the pond. Just be careful that the ice will be thick enough to support your weight. If in doubt, tie a rope around your waist and tie the other end to the tractor. If you fall in just pull yourself out using the rope. If you have a helper, they can pull you out by tieing the rope to the back of the tractor. They just need to be careful to stop once you are out.