Krachai, lesser ginger, fragrant ginger
Gaeng Som Pla ~ Sour Fish Soup, the original uses  pla chon (snake head fish)
It can be substituted with Talapia.  There are many variations to this soup
because of the vary spices, sauces and vegetable.

1½  - 2 lbs fish fillets.
4-5 cups of fish stock ~  
homemade

Group 1
3 tbsp garlic, chopped
3 tbsp chopped shallots
3 tbsp krachai (lesser ginger), thinly sliced
3 tbsp of mixed red and green chile ~ or serrano
1 tsp kapi (shrimp paste)
¼ cup
fish sauce
¼ cup tamarind juice
1-2 tbsp palm sugar
2 cups coarsely chopped *vegetables

1.  If you use fresh whole fish, clean and cut fillets from fish.
Note: Fish parts can be used to make fish stock  

2  Bring stock to a boil.  Add group 1 and return to a boil; add the fish and simmer
until the fish is cooked through.

*Note: You may vary the types of vegetables to suit your taste.
Serrano chile
Thai chile