Like RC51, I've picked a lot of Wisconsin leeches off my body in younger days. Anyway, here are a couple of older links regarding leeches: What to do about leaches Leeches . . . . Good or Bad?
I hate to be a pessimist. But, sometimes things just can't be easily corrected.
As a kid, we used a lot of leeches for bait. One of our family cabins on a lake in Wisconsin still has a very abundant supply of leeches. The lake is well populated with walleye, perch, musky, and several varieties of fat sunfish. Commercial bait dealers trap them in this lake. Many of the lake shore land owners catch them for bait. Everybody takes them out of the lake when they go swimming. In my 60-some years, the leech population on this lake hasn't changed a whole lot.
Catching them in a can full of rotting fish guts will remove a lot of them, but . . .
Once a body of water is infested with leeches, I'm not sure if you can ever completely get rid of them, short of poisoning, draining, and then and bombing the body of water. Bass love them. Walleye love them. Perch love them. I'm sure other fish love them. The leeches love people, and leeches multiply pretty fast.
Sorry for such a negative response. We bought an above ground pool for the grand kids because they didn't want to swim in our pond.