Cut into guavas into halves and using a small spoon, scoop out the seeds.
In a bowl, combine guava seeds and pulp and 1 cup of the water. Using the back of a spoon, mash to extract pulp and strain in a fine-mesh sieve. Reserve the guava juice and discard the seeds. Set aside.
Trim about 2 inches from the kangkong stalks and discard. Cut kangkong into 3-inch lengths, separating the sturdier stalks from the leaves. Wash thoroughly and drain well. Set aside.
In a large pot over medium heat, combine the remaining 6 cups water, onion, and guavas. Bring to a boil.
Lower heat, cover and simmer until guavas begin to soften.
Add fish sauce and the extracted guava juice.
Add milkfish and cook for about 4 to 5 minutes or until almost cooked through.
Add shrimp and cook for about 4 to 5 minutes or until color changes.
Season with salt to taste.
Add kangkong and cook for about 30 seconds or until just wilted. Serve hot.