Have you given any thought to buying some grass carp? About ten would do if you have 50% coverage. It is much safer at this time of the year than trying to use herbicides, especially if you are unfamiliar with them. I have never used weedplex, but diquat of any name is very sensitive to mud so if you stir any up while aplying you will neutralize it. You also must apply it at the proper depths the weeds are growing and in proper fashion as to not kill too much at one time or you may also kill your fish.

-HH