Pour the gallon of water into a crock or other large nonmetallic container. Dissolve the salt in the water, along with the sodium nitrate and sodium nitrite. See whether a chicken egg will float in the solution. If not, add more salt until the egg rises. Then stir in the Liquid smoke. Put the fillets into the solution and weight with a plate or some nonmetallic object of suitable size; the idea is to keep the fish fillets completely submerged at all times. Leave the fish in the solution for 4 days. (If you feel compelled to stir the fish a time or two, use a wooden spoon.)
Drain the fish. Before serving, slice the fillets into very thin slices. Partly freezing the fillets and slicing with a sharp, thin knife will help. Serve atop crackers.