Communication: Female mollies use various body postures and movements to communicate with other fish, including males. Splashing water with her tail could be a way for the female to signal her readiness to mate or to attract the attention of a potential mate.
Defense: If the female molly feels threatened or stressed, she may splash water as a defensive mechanism to deter predators or other fish that are encroaching on her territory or competing for resources.
Territory Marking: Female mollies may also splash water to mark their territory and establish dominance over a particular area in the aquarium. By splashing water, they are essentially leaving a chemical scent that serves as a warning to other fish to stay away.
Playing: In some cases, splashing water with the tail can simply be a playful behavior. Mollies are known to be active and playful fish, and they may engage in this behavior as a form of entertainment or exercise.
It's important to observe the female molly's overall behavior and context to determine the reason behind her splashing water with her tail.