Carp
A rich freshwater fish with firm, slightly earthy flesh, the classic Hungarian choice for fish soup and frying.
In German-speaking countries carp is sold mainly from autumn to spring at fishmongers, markets and fish farms, often to order. In English-speaking countries it is rarely stocked and is most likely found at Eastern European or Asian fish shops.
Substitute:
Catfish, or catfish mixed with zander or pike-perch, comes closest, but it is milder and leaner, so a fish soup loses some depth.