Your question is not that stupid, some fish caneven live out of water on dry land, the Mudskipper and the climbing perch for example spend much of their time out of the water and use modified gills to breath air and pectoral fins that double as legs.
I dont think it's life would be very long in milk or pure water thats had everything taken out of it, like distilled water for that matter.
They could live in diet lemonade and trim milk, but only for a limited time. Eventually the substances foreign to them [fats or sugar] will clog up their gills and they will drown.
lol.. all these answers are tempting me to experiment looooooool..... i wont tho i love my goldfish... they have had a odlly long life.. how long to goldfish life upto? coz i wont both of mine from a fun fair about 10 years ago.....