Prepare Ingredients: Separate the sweet potato leaves from the stalks and wash them thoroughly. Slice half a medium onion, wedge the tomato, peel and optionally slice the ginger, and wash and cut the tilapia into serving pieces.
Boil Broth: In a pot, bring water (or washed rice water for a thicker soup) to a boil. Cover to speed up boiling.
Add Aromatics: Once boiling, add tomatoes, onions, ginger, and chili peppers. Boil for about 3 minutes to infuse flavors.
Cook Fish: Slowly add the tilapia pieces to the boiling broth to avoid splashing. Reduce heat to low and simmer gently for 7 to 9 minutes until the fish is cooked but not soggy.
Add Sourness and Season: Add lime juice to the soup to provide sourness. Season with salt and optionally fish sauce to taste.
Add Sweet Potato Leaves: Add the sweet potato leaves to the pot, cover, and let cook for about 1 minute. Turn off the heat and allow residual heat to finish cooking the leaves.
Serve: Transfer the soup to serving bowls and serve hot, ideally with rice.