In a large, wide pot, bring fish stock to a boil. Reduce heat to simmer.
In a large bowl combine whitefish, yellow pike, and carp. Add onions, eggs, matzo meal, salt, pepper, and 1/4 cup cold water. Stir with a wooden spoon until mixture forms a paste.
Moisten hands with cold water. Shape about 1/3 cup of mixture into an oval slightly larger than an egg (3 to 4 inches long). Place gently in simmering stock. Repeat with remaining mixture. Add carrots. Sprinkle with paprika. Cover and simmer, until firm and opaque, about 1 hour. Cool to room temperature. Transfer to an airtight container. Chill until stock gels, at least 4 hours or overnight. Serve cold, or at room temperature, garnished with fresh horseradish and a sprig of dill. May be made up to 2 days ahead.