In a pot, bring the water to a boil. Add the lemongrass, kaffir lime leaves, and chilies. Let it simmer for about 5 minutes to infuse the flavors.
Add the fish fillets to the pot and cook until they are cooked through, which should take about 5-7 minutes. Remove the fish from the pot and set it aside.
Add the pineapple chunks, bell pepper slices, and tomato wedges to the pot. Let it simmer for another 5 minutes until the vegetables are slightly tender.
In a small bowl, mix the tamarind paste, fish sauce, lime juice, and palm sugar until well combined. Add this mixture to the pot and stir gently.
Return the cooked fish to the pot and let it simmer for another 2-3 minutes to let the flavors meld together.
Taste the soup and adjust the seasoning if needed, adding more fish sauce, lime juice, or sugar according to your preference.
Serve the Kaeng Som Pla Sapparot hot, garnished with fresh cilantro leaves. It's typically enjoyed with steamed jasmine rice.