I once set up a camera on a tripod with a strobe flash to take pictures of my fish-only tank once every two hours for three months. I was condicting an amateur study of fish distribution within a tank. Where they liked to sleep, what currents they preferred, etc. The distribution turned out to be random except for three or four days per month when half the fish would always huddle into one corner of the tank. Upon further investigation it was discovered that these fish were always males.