So I have lived this experiment. Back when I lived on a submarine in the US Navy, when we went out to sea for several months at a time we made all of our own drinking water and I was the person who ran the distillation plant. When you run the plant almost continuously for months straight in the open ocean, it produces cleaner and cleaner water as the plant's essentially continuously flushing itself. After several months, the water in our potable water tanks would pass all the tests to be pumped straight into the nuclear reactor. All the scum that grew inside the potable water tanks would slowly die, as it had no nutrients to live off of. So the whole crew was drinking Type 1 pure water. When new people came onto the boat mid-deployment, they would often get diarrhea, but they'd acclimate and they'd be fine after a few days. That is my (unscientific) eye-witness first-hand lived experience. I wouldn't be surprised if a fish were very unhappy living in pure water, but I seriously doubt a fish would die in ten seconds.