Place cod in a pot and fill with cold water, covering cod completely. Soak at least 12 hours, at room temperature Remove from water and rinse.
Heat oil to 360 degrees F in a deep pot until it comes about 2 inches up the side of the pan. In a medium saute pan, over medium heat, add oil and saute onions, celery, and garlic, until vegetables soften, about 5 minutes. Remove mixture to a bowl and let cool. Add cayenne, scallion, cilantro, lime zest and juice, flour, baking powder, milk and egg. Mix together well. With a fork, flake the cod, being sure to remove the skin. Add the cod to the batter. Using a 1-ounce ice cream scoop, scoop up fritter batter and place into preheated oil. (Fritters should measure about 1 1/2 inches in diameter.) Work in batches. Don't add more than six at a time. Cook until fritters are golden and cooked through, about 6 to 8 minutes. Remove to a paper towel lined plate to drain. Serve with lime wedges.