If the cod has been preserved in salt, then desalting will be necessary before cooking. To desalt the cod, place the fillets into a large bowl and cover with water; place in the fridge. Change the water four times a day over a 48 hour period. If the cod is cut into pieces before desalting, the length of time needed to soak will be reduced. If the cod is fresh and has been unsalted, you can use right away.