I watched Fish pretty regularly and I remember it being a decent show. Similar to Barney Miller in that it was low-key and intelligently written. Not like most typical 70s sitcoms. None of the kids were pushed into the spotlight with any cutesy catchphrases. And Abe Vigoda played a fairly realistic ‘old-guy-who-dislikes-kids’. But my memory might be off. I remember there being a couple cute girls on it, I may have only watched it for that (I was 12-13)!
I think a good reason that Barney Miller never jumped the shark is there were almost no cast changes, which is rare for an ensemble show that ran that long. The only one really was when Dietrict replaced Fish and that turned out to be an improvement. Hardly anyone remembers Chano (and he wasn’t replaced with anybody) and when Jack Soo died they smartly never replaced him with a new regular character. Levitt occasionally filled in for him and I think they tried a woman or two, but he was never actually replaced.