Stir in the sour cream into the small bowl of crema that you have refrigerated. (But omit this step if you are using full-fat yoghurt.)
Toss the carrot and cabbage together to make a slaw.
Warm the tortillas in a skillet or the microwave then lay flat on large plates (2 per plate).
Top each tortilla with one-fourth of the slaw mixture.
Top each taco with one-fourth of the fish. (It might be too much fish. If so, it's great to snack on left over later.)
Top each taco with one-fourth of the crema that you reserved.
Serve with a wedge of lime and fresh cilantro if you have it.