I believe we've already covered this. The saying is a euphemism/idiom for doing something so easy that success is guaranteed.

The phrase 'sitting ducks' refers to an easy target. That isn't what the idiom being discussed here means. The saying 'shooting fish...' is about the ease with which something can be accomplished successfully. Another American phrase similar in meaning would be 'slam dunk'.

This is the sort of thread that takes the joy right out of creative language use...