Because natural fish sauce requires time to make and very fresh, good quality fish, substantial investment is necessary for large-scale production. This has resulted in the proliferation of a number of less-than-pure products. Some are made by the process of hydrolysis in which some kind of enzyme or acid is added to hasten fermentation, while others are made by diluting natural or hydrolyzed fish sauce with salt water flavored and colored with sugar, caramel, monosodium glutamate (m.s.g.), by-products from the production of m.s.g., saccharin, and other natural or artificial flavorings and coloring.
How to tell which brands are good or not? Check the labels, though unfortunately, the certification of quality is not always clearly translated into English; and nutritional analyses cannot be relied upon, as they are outside the scope of many manufacturers who quickly slap these on just to meet U.S. import requirements. Short of being able to decipher or trust the labels, look for fish sauce with a clear, reddish brown color, like the color of good whisky or sherry, without any sediments. If the color is a dark or muddy brown, the sauce is likely to be either a lower grade, or one that is not properly or naturally fermented; it may also have been sitting on the shelf a bit too long. Good fish sauce also has a pleasant aroma of the sea, not an overwhelming smelly fishiness, and should not be overly salty. If the bottle you have been using makes the dishes you cook taste too fishy, try a new brand..