Fish sauce is called “nam pla” in Thai, meaning literally “fish water.”
There are four grades of Fish Sauce made in Thailand.
All will be a reddish-brown sauce; better quality ones will be somewhat transparent, rather than murky. All are generally made from very small fish, such as anchovies.
The fish are caught and, while still very fresh, rinsed, then packed in layers in large clay jars. There will be a sea salt, then a layer of fish, then similarly repeating layers (starting and ending with salt) until the jar is filled. The jar is then covered with a bamboo mat with a weight on it, to press it all down, and let sit in a sunny spot for 9 to 12 months to ferment. The jar is uncovered from time to time to let fresh air in. During this time, the salt draws out the liquids from the fish as the fish decompose.
The liquid is removed either by siphoning or through spigots at the bottoms of the jars. The liquid is then filtered through cloth, and put into uncovered large jars for a few weeks to allow the strong odours to weaken. It is then bottled. The remains of the fish still in the first clay fermenting jars have salt water added to them, and are allowed to sit for 2 to 3 months, for a lesser, second-grade of fish sauce to be produced. This liquid is then drawn off and filtered, then more salt water is added to what’s left in the jars to make third grade Fish Sauce. Finally, the lowest grade (fourth grade) is made by boiling anything leftover in salt water.
First grade fish sauce is sometimes, instead of being sold on its own, added to second and third grade sauces, which are cheaper and more marketable, to improve their flavour.
Some producers add enzymes to speed up the fermentation process. Some dilute the fish sauce with flavoured, coloured water.
Good quality Thai Fish Sauce should be reddish brown. It will smell like the sea, but not overwhelm you with a fish smell or taste. It will still have, though, a pungent smell that diminishes in cooking.
If the fish sauce is very dark or murky, it’s a low grade or not a pure one. Such low-grade Fish Sauces may contain ingredients like hydrolysed wheat protein and fructose. Pure ones will probably just list on the bottle as ingredients something like Anchovy fish extract, salt and sugar.