What is a good substitute for Fish Sauce?
Try mixing hoisin or miso into low-sodium soy sauce. 

From one of my favorite bloggers, Smitten Kitchen, "I often see low-sodium soy sauce suggested as an alternative but I’m not convinced it’s a fair swap. There’s something more caramelized and fermented in the fish sauce that you’d miss. If you feel like playing around, I might whisk some additional hoisin or even miso into that soy sauce for a more complex flavor."