What can I substitute for fish sauce in Thai food?
Soy sauce would not be a good substitute; it would alter the flavor drastically. I cannot think of anything that would give a similar flavor that you would not also be allergic to. You will definitely need to replace the salt missing by not using Nuc Mam (fish sauce). Some spices/sauces that would impart a spicy (but different flavor) are five-spice powder, oyster sauce (if you are not allergic to it), ginger, galangal, kaffir lime leaves, and lemon grass. If you can tolerate the heat from the peppers, you might also increase the amount of peppers or pepper sauce you are using. Too bad you are allergic to the Nuc Mam.
