Larger varieties of fish, such as mackerel and sardines, also make good fish sauce, but because they are relatively more expensive due to their value as a food fish, they are seldom used in the commercial production of fish sauce. For fish sauce to develop a pleasant, fragrant aroma and taste, the fish must be very fresh. 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.
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. 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. Because flavor is substantially reduced with each fermentation, top-grade fish sauce is frequently added to the lower grades to improve their flavor. Because natural fish sauce requires time to make and very fresh, good quality fish, substantial investment is necessary for large-scale production.