Fish takes a short time to cook. Fish belly cut into smallist pieces will cook even faster. Do not overcook the fish based on the assumption that the soup will be more flavorful that way. What you need to spend time and effort in is in sauteeing the shallot and ginger to give them time to caramelize a bit.The chili leaves in the ingredients list are exactly that. Leaves from the chili plant. In our case, that would be leaves from bird's eye chilies.