If fish sauce is not your cup of tea or if you don't have fish sauce handy, you can replace it with light soy sauce instead, although it will lack the unique umami flavor that comes from fermented small fish. You can also try adding a mushed-up anchovy (from canned anchovies) to the soy sauce to add more umami flavor. Since fish sauce is made with fermenting small fish, for those of you who are vegan, there are plant-based vegan fish sauces on the market these days.