For my 18x20 garden pond, I built a retaining shelf about 1’ under the water line so I could place stones and gravel on it, up and over the water line, covering the liner along the way. Pea gravel and smaller rocks to fill the gaps and block UV. The bottom of the pond is basically bare except for a few random large rocks and planters. This allows cleaning it out when necessary which I did this past spring via a large homemade siphon system.
It took a lot of stones, but it looks nice. Regular soils wick too much water out, and planters roots will quickly find their way in. That happens with gravel also, but to a lesser degree and is easier to clean up.
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