Step 1Bring water and lemon grass to vigorous boil in large pot. Boil 5 minutes, half-covered, then reduce heat and simmer over medium heat a few minutes.
Step 2Once broth is simmering, scoop about 1 cup liquid into small bowl. Add tamarind pulp to bowl, stir well to dissolve thoroughly and set aside.
Step 3Rinse off fish. If using large steaks, cut into roughly 1 1/2- to 2-inch pieces. If using small steaks, cut in half. Add to broth along with garlic, bring to a boil, and simmer until fish is opaque, 3 minutes. Add fish sauce and salt. Place sieve or fine strainer over soup and pour tamarind liquid through it. Use back of wooden spoon to press tamarind pulp against strainer, then discard remaining seeds and pith. Stir stew well and simmer several minutes.
Step 4Add tomatoes, green onions, culantro, Asian basil, rice paddy herb and sugar to stew and simmer 5 minutes. Serve hot.