Why is the fry called a tail fish and the adult fish called a fish?

One-tailed fish is a fixed usage of quantifier. It is very vivid to use a certain part of the fish body as a quantifier of fish. A tail fish is a written usage, which has the same meaning as the spoken word "a fish".

It is also said that the fry are too small. In the fry hatchery, there are tens of thousands of fry in one ladle. The fry huddled together and swam desperately, only to see the fish's head missing, so it was called a fish.