From the end of TFA. “If ultrapure water was consumed continually, epithelial cells would be steadily leached (or perhaps instantly in the case of the goldfish

From the end of TFA. “If ultrapure water was consumed continually, epithelial cells would be steadily leached (or perhaps instantly in the case of the goldfish’s respiratory system).” Basically it would scour cells from you digestive system, roughly equivalent to corroding it away. It would probably take a few liters or more, but it would also probably hurt like hell.