In a large pot, bring the water to a boil. Add the lemongrass, galangal, kaffir lime leaves, shallots, Thai red chilies (and optional mild chili powder), and tomato. Reduce heat and simmer for 5 minutes to let the flavors infuse.
Add the mushrooms and mixed seafood (including optional salmon) to the pot. Simmer for another 5-7 minutes until the seafood is cooked through.
Stir in the lime juice, coconut milk, sugar, salt, and stock powder. Continue to simmer for 2-3 minutes.
Adjust seasoning with fish sauce to taste. Serve hot with jasmine rice, garnished with fresh coriander and spring onions.