My grandma taught me how to cook fish soup. Most of the time, she would make it from the heads, tails and stomachs of carp and Carassius. If she ever found caviar in any of the fish, she would put it in the fish soup to cook as well.
Salt the fish a little and sprinkle it with lemon juice, then boil it in salted water. The lemon juice keeps the meat tight. Boil it about 5 min.
Debone the fish and set aside part of the broth. In boiling water, mixed with part of the fish broth (pure fish broth is a bit too strong for my taste, so I dilute it with water), put the vegetable spice, onions, carrots, peppers, potatoes and oil. Once they boil a bit, add the tomatoes as well.
Cook until the vegetables are fully ready. Toward the end, add the pieces of fish.
Enjoy! It's the tastiest soup grandma used to make!
Note: My bowl is about 1 2/3 cups (400 ml).