Answer: Water poppy (Hydrocleys nymphoides) can be a vigorous grower in a small water garden like yours. These plants are often grown in submerged containers in a larger pond. I don't think the water poppies killed the lilies, but they can certainly be tough competition. In your case, I think you'll need to pull out the poppies and replant them in a container to allow more plant diversity.
As to how much of the pond surface they should cover, that is totally up to you. Most people want some uncovered areas in order to be able to see the fish swim by and to allow growing room for other desirable plants. You can also provide shade for the pond with above-ground shrubs and small trees.
For more info on water poppies and other blooming water plants try this web site:
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