Salt fish or salt-cured fish is inedible as it is. Salt fish is a preserved fish with all the water removed. Either with dry salt or with a brine, salt curing was the only widely available method of preserving fish until the 19th century. People still eat salt-cured fish today. But, in order to eat it, you must remove the excess salt and rehydrate it.
There are two main methods to remove the excess salt from salt fish—one way takes an hour and the other requires an overnight soak. This process of removing the excess salt from the fish also rehydrates it. As a rule, the aim is never to remove all the salt from the fish completely; there should always be a salt taste remaining. You want the fish to have some flavor.
Another important factor to consider is that not all fish are the same. Some fish are saltier than others, some retain salt more. It is important to taste the salt fish after the initial stage of soaking or boiling to ascertain whether or not you should add another stage of desalting the fish.
Overnight Soak Method
Place the fish into a large bowl and pour boiling water over the fish. The water should cover the fish. Cover the bowl and let the salt fish soak overnight.
The following morning, drain off the salty water. Remove the bones and skin (if using bone-in salt fish). Pry off a piece of the fish from the meatiest part of the fish and taste it. Do not taste the top piece of the fish that has been directly exposed to the hot water, taste the inner piece. You are checking to see how much the hot water penetrated the saltiness of the fish. If it tastes very salty, add the fish to a pot and pour in hot water to cover the fish. Set it on high heat and bring it to a boil. Let the salt fish boil for 20 minutes; drain, and when cool enough to handle, your fish is ready for you to use in any recipe calling for that type of fish.
One Hour Method
Add the salt fish to a pot of hot water. Let the water cover the fish. Place the pot on high heat and bring it to a boil. Let it boil for 25 minutes. Drain. Add a fresh batch of hot water. Cut up a whole lime or lemon into wedges and add to the pot and bring to a boil. Let it boil for 20 minutes. Drain, and when cool enough to handle, remove the bones and skin (if using bone-in). You are now ready to proceed with any recipe that is calling for that type of fish.
Variations on These Methods
There are some methods that do not call for boiling water at all. One method calls for soaking the salt fish in cold water and placing it in the refrigerator. One recipe calls for this cool soak for one day, another calls for this cool, refrigerated soak for three days. The length of time you need depends on the type of fish you are using, the thickness of the fish, and the overall salinity of the fish itself.