If you are wondering why your water lily have yellow leaves, you should know that every leaf is born, lives and dies.
Yellow leaves are replaced by new leaves. Other healthy leaves will be born, as happens with all plants.
If you have just planted the water lily, normally all pre-existing leaves will quickly turn yellow and new leaves will be born in their place. Make sure you have fertilized it enough.
If you have spare time, you can cut off the yellowing leaves or rotten flowers, however this is not a necessary thing to do. They will generate silt that settles to the bottom, which will act as fertilizer for the water lilies.
You can find a lot of detailed information on how to grow water lilies and obtain a balanced pond in the Guide to Growing Aquatic Plants and in this free Video Course!
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