Cut leek crosswise into 1/2-inch slices and in a bowl soak in water to cover, agitating occasionally to dislodge any sand, 5 minutes. Lift leek out of water and drain in a colander.
In a bowl toss snapper gently with salt and chill, covered.
In a saucepan cook leek with salt to taste in oil over moderately high heat, stirring occasionally, until softened, about 5 minutes. Add garlic, coriander, and cayenne and cook, stirring, 1 minute.
Peel potato and cut into 3/4-inch cubes. To leek add potato, tomatoes with juice, the 1 1/4 cups water, orange juice, and bouillon cube, stirring and crushing tomatoes against side of pan, and simmer until potatoes are almost tender, about 10 minutes. Stir in snapper, parsley, zest, and salt to taste and cook until snapper is opaque, about 5 minutes.
