Anything inexpensive :-) OK, that's a bit obtuse. Since pond plants have their feet in the water all the time, they tend to grow a bit rampant, which means they're typically plentiful and therefore inexpensive. I probably have to clear 1/3 to 1/2 of iris plants from my pond each year for example (keep an eye open for free plants in a bucket in front of someone's house, or on a noticeboard at work. Or ask other pond owners. Most are likely throwing loads away!).
Various reeds, Marsh Marigolds, they'll all fill space quickly and cover the edges of the pond
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