It turns out that ducks and leeches are mutually predatory. Getting rid of the leeches might not be as as simple as buying a couple of ducks.
"Ducks have also been used to control leech populations, but an over abundance of ducks can create other more serious problems.In order to keep ducks near your area for leech control they will also have to be fed.Duck feed is high in phosphorus and travels rapidly through the duck's digestion system.A readily available form of phosphorus is then added to the lakes.If the lake is small and the ducks numerous, enough phosphorus can be added to the lake to cause algae to grow. Ducks are also an intermediate host for a number of parasites that may not be desirable in a swimming area."
https://www.maine.gov/dep/water/lakes/leech.html
I suppose it also depends upon what type of leeches you have. "Muscovy Duck Eye Leeches"
"These leeches are attached to the inside of the ducks' eyelids, but to avoid being scratched off, they are somehow finding a way to attach under their third eyelid."
Then there are duck specific leeches:
https://en.wikipedia.org/wiki/Theromyzon
"Leeches of this genus parasitize birds and are sometimes called duck leeches, although their hosts are not limited to ducks. The Theromyzon species tend to feed in the nasal cavities of waterbirds in general, from ducks to penguins.
As parasites of birds, and in many cases migratory waterfowl, these leeches typically have a broad distribution range. Leeches of the Theromyzon genus have been observed in the Americas, Eurasia, and Africa."
You may wish to try this:
"A measure which can be successful in controlling leeches is bait trapping.A metal can with a reclosable lid (a one pound tobacco or coffee can is ideal) drilled with small holes (depending on the size of the nuisance species) and baited with raw meat may trap large numbers of leeches from a heavily infested area."