Place tamarind pulp in a strainer, pour hot water over it, and let it sit for 5 minutes.
Mash pulp into a paste, and strain into a bowl.
Heat oil in a saucepan, add garlic, lemongrass, onion, and lime leaves.
Cook until garlic browns and onions soften.
Add tomatoes, pineapple, fish sauce, chilies, white pepper, and soy sauce.
Pour in tamarind juice and water, and bring to a boil.
Gently stir in fish and chilies, cover and simmer for 10-15 minutes.
Uncover and serve hot. Garnish with herbs as desired.