Why is the growth of the Jewish people compared to the reproduction of fish (veyidgu lerov), and why do we "feed sins to fish" at Tashlich? One reason given is the idea that the "evil eye" does not apply to fish. But why should that be the case? The usual explanation is that they are underwater and are not seen.
This explanation has an interesting Biblical parallel. In Breishit 2:19 every animal is given a name by Adam except for the fish. Because fish are "anonymous", even if they are seen by people, presumably they are not reckoned and are thus free from the evil eye.
I don't really know what the evil eye is supposed to represent or whether it really exists today. But it is nice to see a clear Biblical source for one of its supposed characteristics, which otherwise would seem to be completely arbitrary.