Thanks for the comments. You are right about the grass carp. They have gotten quite large and I am not sure what they are eating. It seems that our algae has gotten worse since we put them in (3 years ago). I am wondering if they are eating other types of algae/vegetation and now this type of algae is taking over. I'll try your suggestions. I am concerned that the flow is such that it will not stay in the pond long enough. When I use a dye it is essentially gone in 48 hrs.