"Aquatic Use and Considerations

2,4-D is a widely-used herbicide that affects plant cell growth and division. It affects primarily broad-leaf plants. When the treatment occurs, the 2,4-D is absorbed into the plant and moved to the roots, stems, and leaves. Plants begin to die in a few days to a week following treatment, but can take several weeks to decompose. Treatments should be made when plants are growing.

For many years, 2,4-D has been used primarily in small-scale spot treatments. Recently, some studies have found that 2,4-D moves quickly through the water and mixes throughout the waterbody, regardless of where it is applied. Accordingly, 2,4-D has been used for whole-lake treatments.

2,4-D is effective at treating the invasive Eurasian watermilfoil (Myriophyllum spicatum). Desirable native species that may be affected include native milfoils, coontail (Ceratophyllum demersum), naiads (Najas spp.), elodea (Elodea canadensis) and duckweeds (Lemna spp.). Lilies (Nymphaea spp. and Nuphar spp.) and bladderworts (Utricularia spp.) also can be affected."

Rusto, I am not an expert, but I do still use my college chemistry at my real job.

I couldn't find the product label for your bag. However, most of that type of product use 2,4-D as the main broadleaf herbicide.

From the quote above, you can see the 2,4-D is used as an aquatic herbicide. I ran the math on some products similar to yours. A 40# bag of weed and feed contains about a 1/4# of 2,4-D. The maximum treatment rate for ponds is 4# per surface acre of pond.

You could probably dump 16 bags straight into a 1-acre pond. (At least as regards the 2,4-D content.)

This makes sense, otherwise every river downstream of a city with abundant lawns would suffer massive fish kills every spring.

WARNING
This is just one "non-expert's" calculation based on similar products. I think the product should be safe for Rusto to use on the grass around his pond if he follows the product use instructions. Further, the longer it stays on his grass and weeds prior to a rain event, the lower the concentration of the product that will make in into the pond.

There are many herbicides that are highly toxic to fish. I use some of them on invasive trees at my farm, and do everything possible to keep them from entering the water system.