esshup may have a suggestion that involves "Cat litter".. We'll see if we can get him to elaborate. If nothing else, there's always aquacide pellets.
I wanted to use a herbicide to get down to the roots of a plant in the pond, so I used a liquid herbicide and a mortar mixing tub. I threw in about 1/2 bag of clay based oil dry, and mixed in the liquid herbicide. Then let it sit for a day, and broadcast it into the pond in the area that I wanted to kill the weeds in. It worked, but it wasn't a permanent fix, I had to re-treat the area in about 4 months.
Having a flow thru pond to treat, that will be difficult.
It now seems like I have two options. 1. Drag area of pond and add grass carp this fall to keep Widgeon grass under control in following years. 2. Use “cat litter” or oil dry with an herbicide and add grass carp this fall to keep Widgeon grass under control. The concerns I have with option 2 are possibly killing off too much/too little vegetation, lack of DO, fish kill because it is a flow thru pond with 180-220 gallons/minute.
I am also inclined to believe that the drag will bring additional sediment/seeds towards the bottom drain where I can open it to discharge from the pond. What do y’all think?
