Clean the stockpot and place it over medium-high heat. Add the butter and when it has melted add the onions and cook until they are tender but not browned. Add the celery, stir and cook for another minute. Pour in the reserved stock from the quahogs, and add the fish stock and herbs. Bring the mixture to a boil. Immediately lower the heat, cover and simmer for about 10 minutes, partially covered.