The two fish (one male another female) would have to evolve so similarly that the odds against it are probably greater than the number of stars in the universe. In order to breed (and be considered the same species), the DNA must match so closely that it pretty much rules out any real possibility of separate origins. Members of the same species must be in a larger collection that continually shares DNA in order for it to evolve so similarly as well.
If you are right, my PhD is worthless.