Never had much luck with any barley straw products. What kind of algae? Green water, hair/string algae? Do you have any test kits for Phosphates or nitrates? If you've had a good amount of rain lately or even snow this past winter then it could be fertilizer run-off. Search Potassium Permangamate or using peroxide, I don't like adding these things personally and avoid them at all costs, but there are some who have and use them. I could give you a person who does use this stuff and is a highly regarded pond supply salesperson ( so I've heard ) if you wish. He can tell you how much to use, blah, blah, blah etc.

I am no expert, but do not think there are any fish that can control algae. Especially in a natural pond of that size. I would also think that any chemical would be cost prohibitive. I think there are sites for natural ponds that might have some useful ideas. I'm not much help, but I bet someone here will have a link or some better ideas. I would try to google algea in a natural pond? Derek