We had leeches in our pond until I reduced some of the vegetation and added RES. Between those two tasks the leeches have been either reduced significantly or eliminated. I doubt they are entirely gone but I haven't encountered a leech in a long time.

How thick is the vegetation around your pond?

JHAP
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"My mind is a raging torrent, flooded with rivulets of thought cascading into a waterfall of creative alternatives."
...Hedley Lamarr (that's Hedley not Hedy)