Collect a handful of lawn grass, place it into a glass jar with ordinary tap water. Place the jar in sunlight for 1 to 2 weeks. At that time there will be daphnia, copepods, and hydra. The daphnia will multiply quickly with decaying plant material and old water. Any rapid change in water parameters, such as a water change, will temporarily affect their population. If this is tried, please write the experience on this page. Given the tank is outdoor, it should be teaming with copepods. Any medications or salt, previously used in the tank, could affect the daphnia.