So, turns out Koi are really, really bloody sensitive.
The rental house we're in came with two ponds of Koi fish, and a gardener who knew how to take care of them. This gardener was, unfortunately, a Malawian on an expired work visa. He recently got arrested and deported, but he sent his brother to come and work for us. The brother told us his brother'd told him how to do all the things that need doing eg. the garden, fish and swimming pool.
Since the brother started working here the fish's water became more and more cloudy so I asked him if he knew how to clean the ponds on Tuesday and he said yes.
Since then, I have learned that Koi really don't do well with changes in their water. The pH and/or temperature changes of draining and refilling their pond is enough to kill them.
About 26 out of the 30 of them.
You remember the bit about them not being my fish, right? Koi sell for literally thousands of South African Rands each. This has not been a cheap mistake. Not one I'll make again, either; but mostly because I really don't see me ever owning or even living near Koi again.
It has been supremely depressing fishing the dead ones out of the pond with a pool net (they didn't all die at once, at first it looked like most would make it; then that some would make it, now I'll be lucky if the remaining four survive). I am yet to decide what to do with the dead fish. I was sort of hoping a cat would come and eat them in the night but noooo, those darn cats are only around when they're in a frisky mood and you're trying to sleep, or when you forget the car window open and they feel the need to mark some territory.
The house's owner isn't responding to my email, voicemail or various text messages but honestly, who leaves expensive fragile little fish with tenants; tenants to whom he doesn't explain anything about keeping Koi, at that; and leaves an illegal worker as the only person who knows how to keep them alive? I have no sympathy. Or maybe I'm looking for someone else to be mad at so I'm not so mad at myself; one of the two.