Wash and clean fish and make slits on both the sides. In a blender add chilli powder, pepper powder, ginger, garlic, turmeric powder, mustard seeds, lemon juice, curry leaves and salt. Blend it well and make a paste. Apply the paste all over the fish into the slit too. Keep marinated fish for 30 minutes. Heat coconut oil in a frying pan, add the fish one by one. Fry until the both sides are crispy and golden brown color. Drain and serve with rice.