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I'm guessing by "nasty" you mean it's very green and smells pretty awful? If so, fish aren't going to help, as there aren't pond fish that can deal with microscopic algae suspended in the water. How deep is your pond? If it's deep enough, you can use underwater plants to pull out all that excess nitrogen (so the ducks will end up just keeping your plants trimmed, instead of eating them outright.) Surface plants like hyacinths would work too, but the ducks will tend to go through them like a hot chainsaw through butter; so you will be replacing them a lot. If pond plants are consuming all that fertilizer your ducks are depositing in the pond, there will be less for the algae; so you will have clearer water as a result.
Otherwise, you're going to have to somehow get electricity down there and drop in a suitably-sized pump and filter setup to circulate and clear the water.
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Oh they'll keep it clean, alright. Any plants, insects, and small fish within easy reach will be cleaned up by the ducks and returned to the pond as organic sludge and ammonia. The bacteria in the water will convert the ammonia into nitrates, but then you end up with nasty green water as the pond explodes with algae blooms from all the free fertilizer.