Place the fish trimmings in a large soup pot along with the vegetables, herbs and 2 qts of water. The water should cover the trimmings. Cover, bring to a boil and then lower the heat and simmer for 30 minutes.
Put the fish fillets along with the onion and garlic through a meat grinder. (a food processor or blender may be used provided you to it in small batches).
Put the ground fish mixture in a large bowl. Mix well and then add eggs matzo meal, ice water, salt and pepper. Combine and then refrigerate.
Strain the fish stock and remove the trimmings and vegetables. Place clear stock in a clean soup pot. Add the sugar, additional carrot, salt and pepper. Simmer gently over low heat while you form the dumplings.
With wet hands, shape the fish mixture into egg sized dumplings and gently lower into the simmering stock. Cover and simmer for 1 1/2 hours.
Let the dumplings cool in the stock until they are room temperature. Remove to serving bowl and arrange to your liking. Scoop the carrots out of the stock and arrange over the dumplings. Pour as much fish stock over the dumplings as you wish. Chill overnight and serve with grated horseradish. The fish stock should gell overnight.