Bring 6 cups water to a boil in a pot. Meanwhile, peel and devein the shrimp, reserving the heads and shells. Throw the heads and shells into the boiling water and set the meat aside. Simmer over medium-low heat for 20 minutes. Remove from the heat and let sit and stew for another 20 minutes. Strain the broth into a clean pot lined with cheesecloth. Add the lime juice, fish sauce, chili paste, raw sugar, lime leaves, chiles, lemongrass, galangal and cherry tomatoes to the shrimp broth and bring to a boil. Reduce the heat and let simmer for 10 minutes. Add the mushrooms, tilapia, clams and reserved shrimp and cook until the clams have opened, about 5 minutes. Discard any unopened clams and the galangal. Divide the soup among bowls and enjoy.