In the 1910s, physiologist Carl von Heß and zoologist Karl von Frisch studied color vision in fish. In what follows, I first show what types of control are involved in their experiments. I then address a rarely discussed issue—the fact that, in these experiments and many others, the animals must be stimulated in order to actively participate. I thus distinguish between two types of control, environmental and behavioral control of activities. Finally, I make a point that goes beyond the problems of experimental control: in studying sensory abilities, it is impossible to directly determine the content of perception. In this sense, these experimental results are, in principle, difficult to control.