Knowledge about the stiffness of fish fins, and if stiffness is modulated during swimming, is important for understanding the mechanics of a fin's force production. However, the mechanical properties of fins have not been studied during natural swimming, in part, because of a lack of instrumentation. To remedy this, a vortex generator was developed that produces travelling vortices of adjustable strength which can be used to perturb the fins of swimming fish. Experiments were conducted to understand how the generator's settings affected the resulting vortex rings. A variety of vortices (14 to 32 mm diameter travelling at 371 to 2155 mm s(-1)) were produced that elicit adequate responses from the fish fins that can help understand the fin's mechanical properties at various swimming speeds (0 to 350 mm s(-1)).