The phrase "rotten fish" refers to something that is no longer fresh, good, or trustworthy. It suggests that something or someone is not reliable or credible, similar to spoiled or decaying fish.
Last year you nearly died from eating rotten fish. (bookbrowse.com)
When each of your books garners nearly a 40 - 50 % 1 or 2 star rating rate, I think we can all agree that we're smelling rotten fish in the dumpster... not just an entree that hasn't found a target audience. (russellblake.com)