You can use whitefish such as cod, haddock, tilapia, sea bass, barramundi, monkfish, or snapper. Salmon also works for this recipe. Stay away from fish that is very lean (like tuna or swordfish) as it won't flake nicely, and very delicate fish (like sole and flounder) as it will fall apart in the soup and practically disintegrate. However, if you have some delicate fish you need to use up, go ahead and throw it in - even if it doesn't hold its shape, it will add flavor and texture!