Note that water spinach and Chinese spinach (amaranth) are two different vegetables. They can be used interchangeably, they taste somewhat similar, but Chinese spinach is so much more flavorful. For the best Chinese spinach egg drop soup, do not substitute water spinach.
Rinse the Chinese spinach well then shake off excess water.
Cut off and discard the root ends of the Chinese spinach.
Thinly slice the Chinese spinach stalks and leaves.
Stir the tapioca starch into the fish broth and heat in a pot. Stir occasionally until just beginning to simmer. By this time, the broth should have thickened and the cloudiness should have disappeared.
Drop in the thinly sliced Chinese spinach and press lightly into the broth. Allow to boil gently, stirring often, for about five minutes.
Stir in the shrimps and mushrooms, and cook, stirring occasionally, for another five minutes.
Taste the broth, and add fish sauce and pepper, as needed.