I get good results with one gallon per acre Reward on duckweed but two gallons per acre is not an uncommon application rate. It depends on the water chemistry and turbidity. Diquat is toxic to fish in soft water at 245 ppm. If your pond is 3 feet deep on average putting 2 gallons of Reward into your pond would be about 1ppm active diquat. Thats with mixing it very well, in reality most of the diquat will be bound by dirt particles. So you see it would take quite a lot of Reward to kill fish with diquat. (about 400 gallons in a pond your size if my quick figures are right)
When treating duckweed with Reward think of it like mowing the yard. One or two times per year wont look very good. keep an eye on the duck weed and treat accordingly. Dont let it cover the pond or you will be asking for oxygen problems after the treatment. As long as its not covering more than 20 % of the ponds surface you can spray weekly. If you get a thick infestation you will need to wait several weeks between treatments. allowing the weeds to fully rot away before spraying again.
with such a small pond you may consider an aerator to help with oxygen
You could probably get away with one pint to one quart Reward per gallon of water for sucessful control. You can usually find a surfactant at any local feed and fertilizer. Look for 90 % surfactant. You dont absolutely need surfactant but it will help.
thats my take
