For fish sauce to develop a pleasant, fragrant aroma and taste, the fish must be very fresh. As soon as fishing boats return with their catch, the fish are rinsed and drained, then mixed with sea salt – two to three parts fish to one part salt by weight. They are then filled into large earthenware jars, lined on the bottom with a layer of salt, and topped with a layer of salt. A woven bamboo mat is placed over the fish and weighted down with heavy rocks to keep the fish from floating when water inside them are extracted out by the salt and fermentation process.
The jars are covered and left in a sunny location for nine months to a year. From time to time, they are uncovered to air out and to let the fish be exposed to direct, hot sunshine, which helps "digest" the fish and turn them into fluid. The periodic "sunning" produces a fish sauce of superior quality, giving it a fragrant aroma and a clear, reddish brown color.
After enough months have passed, the liquid is removed from the jars, preferably through a spigot on the bottom of the jars, so that it passes through the layers of fish remains; or by siphoning. Any sediments are strained out with a clean cloth. The filtered fish sauce is filled into other clean jars and allowed to air out in the sun for a couple of weeks to dissipate the strong fish odors. It is then ready for bottling. The finished product is 100-percent, top-grade, genuine fish sauce.
Second and third grade fish sauces are made by adding salt water to cover the fish remains, letting sit for 2-3 months each time, then filtering before bottling. Finally, the fish remains are boiled with salt water, then strained out and discarded, to produce the lowest grade fish sauce; or they may be added to other fish remains from the first fermentation in the process of making second-grade sauce. Because flavor is substantially reduced with each fermentation, top-grade fish sauce is frequently added to the lower grades to improve their flavor. In fact, many manufacturers do not market top-grade, 100-percent fish sauce, saving it instead to mix with second and third grade sauces in order to produce larger quantities to sell that can still qualify as genuine fish sauce.