Freshwater fish (pike-perch, pike or trout) go well with fresh, delicate champagnes, whose effervescence contrasts with the butter or cream in the dishes.
Sea fish (sea bass, sole, red mullet, Saint-Pierre, turbot) call for more balanced or consensual champagnes. Above all, it's the sauce that determines the choice: a light sauce, a lively wine; a rich sauce, a more mature wine.