> I can't see how further purification could possibly suddenly tip it from "just fine" to "so toxic it will kill a goldfish in seconds" First of all plopping a

> I can't see how further purification could possibly suddenly tip it from "just fine" to "so toxic it will kill a goldfish in seconds" First of all plopping a gold fish just like that into a new tank sounds bad. It is possible the temperature shock killed it. My other guess would be the lack of oxygen. Ultrapure water used in litography is degassed to prevent the formation of small bubbles. No gass, no oxygen, that can also easily kill a fish.