Place the fish in a baking dish and sprinkle with salt, pepper and chili powder. Add the oil and lime juice and rub into the fish until evenly coated. Let marinate for at least 15 minutes, or overnight in the refrigerator.
Brush a grill pan with oil and heat it, on medium heat, until very hot.
Add the fish and cook one side until you see grill marks, about three minutes. Flip and cook on the other side until white and opaque, another two to three minutes. It’s okay if the fish breaks apart while flipping. Repeat with all the fillets.
Warm the tortillas in the oven or on a skillet while the fish fries.
Spoon tartar sauce into the middle of a warmed tortilla, place fish filet on top, then add cabbage, tomatoes, jalapeno, onion, and cilantro. Fold in half and serve immediately.
You may want to use two tortillas for each taco, since they are soft and tend to break apart easily.