I've noticed that fish sauce does not age well here. I keep it in the fridge, but after a few months it gets darker and has more pungent fishy smell and taste. I have to readjust the amount I use, using less than when it's new and completing with soy sauce to balance it out. This is with various different brands of fish sauces (including red boat 40N).
The biggest issue I have with it is that when it's that pungent, when I use even just a few drops of it, it smells horrible in the whole house. Whereas when the fish sauce is new, it doesn't smell as much AND the smell is much more subtle and refined, not as gross.
Is that normal? Have you guys noticed that also? Do you just use up all your fish sauce within 1 or 2 months?