The tail of most fish has a cutout in the middle, but why?

The tail fin of a fish can have various shapes, but in most cases it is divided into two lobes. Moreover, they can be both symmetrical and different in length.
This is due to the characteristics of swimming and the speed of movement of the fish. When swimming, her body is flown around by counter streams of water, which at a certain speed can form real vortices. Therefore, the tail blades must protrude beyond these vortices, otherwise the speed will drop. This is easy to observe with fast and slow swimming fish. The slower the fish swims, the less pronounced the lobes of its tail, and vice versa. The higher the speed of movement, the deeper the notch in the tail will be, for example, like a herring.