Peel the prawns, leaving the tail intact, but DON'T THROW AWAY THE HEAD AND SHELLS. Slice a shallow slit along the back of the prawn and remove its intestinal tract (black line). Optional - for presentation leave a few prawns unpeeled to place on top of the soup before serving.
In a large saucepan, place in the prawn heads and shells (not the peeled prawns), garlic, onion, galangal, lemongrass, kaffir lime leaves and chillies. Pour in the chicken stock and water and place over medium heat until the soup boils.
Once the soup boils, reduce the heat to low and place a lid over the saucepan. Cook for 30 minutes covered for the soup flavour to develop. Using a large heatproof strainer, strain the soup into another large saucepan. Discard the shells, spices and any solids.
Place the tomatoes, sugar and fish sauce into the strained soup and cook over medium-high heat. Once the soup boils, add in the crab, peeled and any unpeeled prawns, cook for a minute then add the mussels, fish and squid. Let the soup cook for another minute, then reduce the heat to low and cook for a further 5 minutes.
Remove the saucepan from the heat and stir in the lime juice. Have a taste and adjust to your liking, if needed, by adding more sugar, salt or lime juice. Give it a good stir and serve with coriander over the top.