Heat the oil in a large pot over medium heat. Add the mustard seeds and curry leaves. Cook until the seeds begin to pop.
Stir in the onion, garlic, and ginger. Cook for 4 to 5 minutes until the onion softens.
Add the turmeric and chili powder. Stir for 30 seconds, then add the potatoes. Cook for 2 minutes to coat them with the spices.
Pour in the coconut milk, water, and tamarind paste. Add salt and bring the mixture to a gentle simmer. Cover and cook for 12 to 15 minutes until the potatoes are tender.
Add the fish pieces. Simmer uncovered for 5 to 7 minutes until the fish is just cooked through and flakes easily.
Remove from heat and sprinkle with chopped cilantro before serving.