Place coriander, cayenne, turmeric, cumin, salt, and lime juice on a large plate and stir to combine. Add about 6 tbsp. water and stir to form a thick paste. Sample the mixture for a good balance of saltiness and sourness; it should be fairly salty at this stage. Rub both sides of fish steaks with paste and set aside to marinate for at least 15 minutes. (This dish can be prepared up to this point as long as 3 hours before cooking. If you marinate the steaks longer than 15 minutes, refrigerate them.)
Rub the bottom of a large, heavy frying pan with vegetable oil and place over medium-high heat. When pan is hot, fry 2 fish steaks until nicely browned and just cooked through, 6-8 minutes per side, turning once. Transfer to a heatproof plate and keep warm in a low oven. Repeat this process with remaining 2 steaks, adding more oil to the pan if necessary. Garnish fish steaks with lime wedges.