Dan,

I am NOT an expert on this topic, but you may be overly cautious. (Which is clearly the best side of the line to be on - when you don't know exactly where the line is.)

They do spray Roundup directly on corn and soybeans, and it is generally safe to use in a backyard vegetable garden where you spray the soil with Roundup and then plant after all of the existing weeds are dead. Once Roundup hits soil, it becomes pretty immobile and starts breaking down fairly quickly.

Maybe you should grab a water sample right after the WORST possible scenario occurs. For example, the neighbor sprays Roundup and it rains 2" that same night and water goes directly into your pond.

You should have a state agency that will test water (generally from water wells) to determine if it is fit for human consumption. You might have to pay a little extra and check a box that your water could have inputs from nearby agricultural production.

If there is no state testing available, then a private lab can run your sample.

Either way, ask whichever lab for their proper collection protocols.

Hopefully, a clean water test will give you some peace of mind on your fish consumption safety.

P.S. Culling some fish is a necessary part of managing a pond. I had a friend's pond that was overpopulated with stunted crappie. He showed us the spot on a hill to dump all of the small crappie that we caught where the vultures subsequently cleaned them up. It was tough for me to just "waste" the fish, even though I knew it was the correct thing to do. You can certainly manage your pond by catching and culling to keep your populations at the appropriate balances. However, catching and eating has definite enjoyable benefits - if you are comfortable eating the fish from your pond!

Good luck on getting your pond into great shape to meet your future goals!