Scoop off 1/4 cup cream from top of canned coconut milk. Reserve remaining cream and milk for another use.
Heat oil in wok over moderate heat until hot but not smoking. Add mustard seeds and cook until they make popping sounds, about 15 seconds. Add ginger, garlic, and chiles to taste and cook, stirring, 1 minute. Add coriander, chile powder, turmeric, and curry powder and cook, stirring, until fragrant, 5 to 10 seconds. Add tomatoes and cook, stirring, 1 minute. Add water and bring to a boil. Reduce heat and simmer, covered, 5 minutes. Stir in coconut cream until incorporated, then add fish and salt and simmer, uncovered, stirring occasionally, until fish is just cooked through, 3 to 5 minutes. Gently stir in mango.