Apparently the down stream fish evolved into a new breed of fish. This demonstrates the weakness of which hypothesis?

So a fish "evolved" into a fish? That's not the claim of Darwinism (a term widely recognized in the scientific community as a valid description of evolution), and it's not the way that "evolve" is typically used. "Evolve" has a much more loaded meaning.

The Bible is fully consistent with fish "evolving" into other kinds of fish. However, Darwinism teaches that fish evolved into other kinds of animals, something completely unproved by science.