Heat the oil in a large pot, when hot, add the garlic and ginger and saute until fragrant, about 1 minute, then add the tomatoes plus juice, sugar, salt, black pepper, and red pepper. Cook and stir for about 3 minutes.
Add the stock and potatoes, bring to a boil then reduce to a simmer. Cook for 10 minutes or until the potatoes are just fork tender.
Stir in the fish, cover, and cook for 5 minutes or until the fish flakes with a fork. Taste and adjust salt and pepper to your liking.
Sprinkle with parsley, serve.
Using Pangasius fillets, which are boneless and skinless, 1 kilo is about 128 Baht. For 4 servings, this is about 95 cents per serving.