Preheat a large pot. Add in the canola oil. Add in the garlic and ginger and stir-fry until fragrant, about 10 seconds. Pour in the fish stock and coconut milk and bring to a boil. Add the lemongrass stalk, kaffir lime leaves and red curry paste. Lower the heat and let it simmer for about 10-15 minutes
Add in the lemon juice and basil and let it simmer for another 2 minutes. Add in the fish fillet and shrimp and cook until the fish fillets turn opaque and the shrimps are pink, won't take very long, about 1 minute or so
Turn off the heat. Ladle the soup into individual serving bowl and garnish with some basil