Drain out some water, don't just pour new water in it. For 1,000 gallons, this would be 200 gallons. Then add fresh 200 gallons with baking soda and sea salt (a handful, not more, but it tends to boost their immune system.) This you should do.
Can you put umbrellas up, anything that provides shade and is in accordance with the property owner and looks like normal patio furniture?
Kois are pretty hardy fish, essentially, and I think the problem you have has build up over some time and resulted in a deadly crack-down of the biological balance, given there's wasn't a jerk pouring soap into it - but the resolutions are the same. But reading your posts, I think it's because of having a fairly new pond with the eco-system not yet in charge, and if you don't have a filtration system and rely on rain water, that doesn't help, I'm afraid. Rain is acidic, having a pH around 5.0, where Kois need to have pH 7.0 - 7.5. They also need a certain hardness of the water, which will buffer the pH and keep to maintain it.
Full sun is to absolutely avoid. Not only because Kois sunburn lightly, but because it will heat up the water, making it prone for crazy pH-changes and rising nitrate levels (which I think has a part in here, too.)
If the chlorine in the tap water is minimal, you can do a partial water change. Fill a few buckets, drop in some baking soda until you have some on the bottom not resolving anymore and pour it in from a height so it aerates the water on itself. You can also sprinkle the baking soda directly over the pond.
How warm is the water? It should never exceed 77° F; this is hard on the Kois and hard on the water itself.
Do at least the water changes you can do. I'm a bit concerned about the other Kois not surfacing. Oxygen at the bottom is the weakest.
I'm crossing my thumbs, because Kois should be happy:
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