I think I can see the confusion about い here. Not so much う お instead of さかな though...
い adjectives are said to have a built in copula, at least in material I've read, and since な in な adjectives is from the classical Japanese copula にある, one could surmise that い is also derived from a copula. As far as I can tell that is entirely wrong, but it is the kind of thing I wondered immediately after hearing that な and だ were derived from forms of ある.
Disclaimer: very little actual understanding of Japanese.
い adjectives are said to have a built in copula, at least in material I've read, and since な in な adjectives is from the classical Japanese copula にある, one could surmise that い is also derived from a copula. As far as I can tell that is entirely wrong, but it is the kind of thing I wondered immediately after hearing that な and だ were derived from forms of ある.
Disclaimer: very little actual understanding of Japanese.