I used a digital camera to film the ripple tank at 240 frames per second with a stopwatch visible. When played at normal speed it takes so long to play all the frames that the effect is slow-motion. Even that wasn't quite slow enough so I resorted to Windows Media Player. Right click and choose Enhancements and then Play Speed Settings.
That gives the control window shown top left. I moved the slider along to a very slow speed. When the film is paused, the forward and back arrow functions will move through the film one frame at a time. I paused the film and wrote down the time shown on the stopwatch. With the film moving slowly, I traced the movement of one wave from the top of the ruler down to the bottom of the ruler. I then paused it again and wrote down the time from the stopwatch. It took 0.91 seconds to move 15 cm. Speed = distance / time = 0.15m / 0.91s = 0.16 m/s. Comparing the wave pattern to the ruler, I think that the wavelength is 12mm. The wave equation is wave speed = frequency x wavelength. So frequency = wave speed / wavelength = 0.16m/s / 0.012m = 14 Hz. It is harder to measure wavelength precisely because the price to pay for more frames per second is lower resolution on each picture. They are more blurred and less bright.
