There is a much easier way to make algae grow in a shallow slow flowing body of water, just paint the bottom a dark color, that way the water stays warm, most algae love warm water. So all you need to do is get a company to come drain it and paint it a dark color, and Bob's your uncle. It has the added effect of the water staying at the ideal temp for algae growth for a longer percentage of the year, just got to find someone stupid enough to sign off on the plan.