There is a better answer for string algae on the top post of this page... Ponds & Lakes

I don't think there's a real answer to your question. Lots of people have experimented with hydrogen peroxide at various levels, but there are so many factors that come into play. People tend to be more concerned about at what level they will be causing harm to their fish or plants. We've used as much as one quart per thousand gallons with no harm to fish or plants in our pond.