Give it a try! Buy about a yard of heavy copper wire, cut it into two or three pieces, bend them into crude circles, and drop them into the pond, approx equidistant from one another. I did this with a 15” scrap of heavy copper wire from my local Ace and it worked like a charm in a stone birdbath of approx 18” diameter. I don’t know the exact gauge, but it was thick enough that I could just barely bend it with my hands. Anything made of copper should work — tubing, wire, pennies (the old ones), even some sort of decoration or statue made of copper.
But what about the fish?...............