Tom Yum Pla (ต้มยำปลา)

The Soup With Rules

ต้มยำปลา is the fish version of Thailand's most famous soup, and because fish behaves differently from prawns, every Thai source leads with the same two rules. The first, from Wongnai, is absolute: ใส่แล้วห้ามคนนะคะ ไม่งั้นปลาจะคาว — once the fish goes in, do not stir, or the soup turns fishy. The second, from TrueID, is about the lime: squeezed into a boiling pot it turns bitter, so it goes in off the heat, or the soup is ladled into a bowl and seasoned there.

The anti-fishy work starts before the pot, as Kapook spells out: buy fish with clear eyes, red gills and firm scales, rub it with salt and rinse it two or three times, skim the foam as it boils — and again, do not stir until the fish is cooked. TrueID's tilapia writer adds her own count: the water must reach three boils — one for the herbs, one when the fish lands, one to finish — and the lid stays on through the whole middle stage.

Then there are the two schools. The classic is น้ำใส, the clear soup, sour from lime. The น้ำข้น (thickened) version stirs nam phrik phao — roasted chilli paste — into the broth, sometimes with a splash of milk, seasoned bowl-by-bowl at the table as Bloggang's ป้าอิ๋ว does it. A third, homier route is Wongnai's tamarind-pot: the sour comes from tamarind squeezed into the water before the fish, with lime still added at the end.

The Recipe (clear-broth version, from TrueID Food)

The tilapia home pot (TrueID): 1 whole tilapia (about 1 kg) cut into pieces, ½ cup lemongrass, 1 cup young red galangal, ½ cup torn kaffir leaves, one lime-sized ball of aged tamarind, 8–10 fresh red chillies pounded coarse with 4 garlic cloves, 3–4 coriander roots, 2 tsp seasoning powder, ½ tsp salt, ½ tbsp fish sauce, ½ ladle pla ra and 3½ cups of water. The creamy bowl (Bloggang): for 2 bowls — 250 g fish fillet, 10 straw mushrooms, ½ onion, a two-finger piece of galangal, 10 big kaffir leaves, 1½ tbsp nam phrik phao, ½ cup plain milk, lime, fish sauce and 15 pounded bird's eye chillies, seasoned bowl-by-bowl.

The Method

1. The herbs. Boil the water hard and add the galangal, lemongrass and shallots. Simmer until the broth smells of the herbs — TrueID simmers them together before anything else goes in.
2. The fish — and do not stir. At a full rolling boil, drop in the fish pieces and leave them. Skim the foam, but never stir: stirring is what makes the soup smell fishy, Wongnai warns. Seabass takes about 3–5 minutes.
3. The mushrooms. When the fish is firm, add the mushrooms and cook until done.
4. The aromatics. Add the torn kaffir leaves and bruised chillies, season with the fish sauce, stir gently once and turn the heat off.
5. The lime, off the heat. Only now add the lime juice — into a boiling pot it turns bitter. Scatter the sawtooth coriander over the top and serve hot, with rice.

Where Thai Sources Disagree

How Western Versions Get It Wrong

• Stirring the pot. The single most repeated rule across Thai sources: fish in boiling tom yum must not be stirred. Stirring breaks the flesh and releases the fishy smell — western recipes casually stir everything.
• Boiling the lime. Lime added to boiling liquid turns bitter. Every Thai source either kills the heat first or seasons bowl-by-bowl; western versions squeeze the lime in mid-cook.
• Simmering the fish to shreds. The fish gets 3–5 minutes at a hard boil and no more. A slow western simmer produces disintegrated fish and a cloudy soup.
• Skipping the foam skimming. Kapook's anti-fishy method is explicit: skim the foam constantly while the fish boils, and the soup stays clean.
• Treating it as prawn tom yum with fish swapped in. Prawns and fish are cooked differently — fish needs the rolling boil, the lid, and the no-stir discipline. The herbs and the lime discipline are the same, the heat management is not.

UK Ingredient Notes

• Seabass: standard in UK supermarkets; tilapia is common frozen at Thai grocers. Any firm white fish works.
• Straw mushrooms: canned at Thai grocers; oyster mushrooms from any supermarket are the closest fresh stand-in.
• Nam phrik phao (roasted chilli paste): jars at Thai grocers — this is what gives the creamy version its red oil.
• Sawtooth coriander is seasonal and hard to find in the UK; ordinary coriander is an accepted garnish.
• Pla ra (fermented fish) for the tilapia home pot is bottled at Thai grocers; leave it out and the soup is still authentic, just brighter.

Sources

This recipe is translated from the following Thai-language sources. Where they disagree, we've said so.