Since its a koi pond and pool, can you tell if there is a bubbler or fountain pipe in the middle? They could be for a pump attachment for the pond or a slide for the pool.
Those end caps look like threaded caps. It's possible they may simply screw off and are for a liquid feeding system(looped back to pond) or water feature beside the pond. Multiple possibilities. Post some pictures of the rest of the plumbing set up and the pond itself.
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