Prep the fish: Rinse the fish well under cold water and remove the scales and guts if needed. If using whole fish, cut it into large serving pieces. Rub lightly with salt, then rinse again to help reduce any strong fishy smell.
Simmer the aromatics: Add the water to a large pot over medium heat. Stir in the galangal, lemongrass, kaffir lime leaves, garlic, shallots, and tamarind pulp. Bring to a gentle simmer.
Season the broth: Add the fish sauce, salt, and MSG. Let the broth simmer for about 5 minutes until fragrant and slightly tangy.
Cook the fish: Gently add the fish and tomatoes. Simmer until the fish is fully cooked and flakes easily with a fork. Avoid stirring too much so the fish stays intact.
Add the herbs: Turn off the heat and stir in the green onions and cilantro.
Serve: Ladle the soup into bowls and serve hot with rice.
Notes
Use the nutrition card in this recipe as a guideline.
Firm white fish works best here because it holds together in the broth. I used tilapia, but snapper, catfish, cod, or sea bass all work well.