A pond system includes a single-piece drop-in flexible liner sized and shaped to partially suspend within a rigid sidewall, creating a cavity that may function as a liquid reservoir. The single-piece drop-in flexible liner includes a base portion surrounded by a sidewall portion configured to rest adjacent to an interior-facing surface of the rigid sidewall. The single-piece drop-in flexible liner further includes multiple tensioning tabs each with a fixed end attached to the base portion and a free end opposite the fixed end. Each of the multiple tensioning tabs is configured to extend under the rigid sidewall and radially outward from the base portion. The pond system further includes a means for securing the free end of each of the multiple tensioning tabs at a location external to the perimeter enclosed by the rigid sidewall.