Farm pond weed control seems to the the hot topic - when it is usually too late to do much about it without affecting fish populations. One of the keys it to begin early, so decaying algae doesn't affect oxygen levels in the water, leading to a fish kill. If you already have had a significant algae bloom, treat smaller areas and allow ample time between treatments.

A good guide for pond weed control can be found at the following link. This has been provided by Mike Roegge Extension Educator.

http://web.extension.illinois.edu/lms/downloads/13075.pdf