Answer: Ponds are self contained systems. The lily pads shade the water and help reduce algae by blocking light and keeping the water cooler, they also use up nutrients in the water and help prevent algae that way. There should be underwater plants that provide oxygen (good for fish) and also filter the water naturally by using up nutrients (eg fish waste, naturally occurring rotting plant material), again reducing algae. If the pond is in good balance, the water will be slightly murky beginning about ten inches down.
Sometimes a pond continues in good health with very little care, especially without fish in it. I would suggest you watch and see how it does before making any changes.
In the fall, remove any frosted plant material such as lily pads and stems and clear out any autumn leaves that may fall into it. Beyond that, you might need to fertilize the water lilies using water lily fertilizer tablets, but only if their blooming is reduced, and only from spring through fall. Read and follow the label directions on how to do that. You do not want to overfertilize or feed when it is not necessary as this introduces excess nutrients into the pond and would encourage algae growth.
In your question you did not mention if the pond has a filtration system or any type of circulation system such as a fountain. If so, you will need to periodically clean the intake on the pump and do routine maintenance, checking the impeller and so on.
Enjoy your pond!
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