I would not use chemicals to control algae. I do not know if your algae control has copper in the ingredients like some do, but if it does, you must be very careful, because copper can be very toxic to fish. Just use a rake to pull out string algae. String algae itself is not harmful to fish, it is a good springtime food for them in fact. I have heard of algae being toxic to the pond is when your kh (buffering) is low and when plants give off carbon dioxide at night it can turn into carbonic acid, if there is not enough buffering in the pond, this causes large PH swings when the sun comes out and the plants start giving off oxygen which raises the PH. Swings in PH are bad, stable PH is good. Algae products, make me nervous, so if you see sickness occur in the fish, it could be from the algae treatment, it so a large water change would be needed. Hope this helps.