Some recipes call for homemade Fish Stock, shown (see recipe below). For a quick substitution, you can use the instant dashi found in Oriental markets. Instant dashi is like instant bouillon. Either mix one 0.35-ounce package with 3 cups of hot water or prepare the dashi according to package directions.
If you don't have enough fish trimmings to make homemade Fish Stock, either ask for some at your fish market or freeze your own trimmings until you have enough. To freeze the trimmings, wrap them in moisture- and vaporproof wrap. Label and date the package, and store it at 0 degrees or lower for up to 3 months.
When fish is cooked in liquid, it gives the liquid a milky appearance. If you want your soup or stew to have a clear broth, poach the fish separately in a skillet or in another saucepan and add the fish to the soup or stew just before serving. If appearance isn't critical and you prefer to streamline the preparation, then cook the fish in the broth as directed.
When cooking fish in soup, small 1-inch pieces will cook faster than whole fillets or steaks. As a general guide, when cooking pieces of fish in soup, allow about 2 minutes per 1/2-inch thickness of the fish pieces.
To store leftover soup, simply place it in a covered container in the refrigerator for up to 2 or 3 days.
Reheat the soup over medium-low heat just until heated through. Be careful not to overheat it because the fish will overcook; cornstarch- or flour-thickened soups will also become thin in consistency.