Lab Water Temperatures: What’s Going On?

The graph that you see at the top of this post shows the water temperatures in the shellfish lab's two tanks from 3 PM on June 5 to 3 PM today, June 12. Since the tanks get water from the same pump, the temperatures go up and down together. That's always been true until today. Between 2 and 3 AM, the temperature in Tank 2 began increasing slowly and started dropping in Tank 1. Then, at 7 AM, Tank 1 started warming quickly while Tank 2 continued its gradual increase. By 3:30 PM, there was a four-degree difference between the tanks! What was going on?