I got up early yesterday morning to tend to some tank chores. I had some small bits of cheato that had got past my sump divider. I stuck my hands in the water and I thought it was a little chilly. I looked at my AC3 and it read 78.3 degrees. I didn't think to much of it at first. So, as I continued on cleaning the sump out my cooling fans over the sump started clicking off and on. I took a closer look at the controller and the temperature started to swing, 78, 77, 76.... It went up and down and up and down. My suspicion started to rise. So, I grabed a water jug and scooped out about a half gallon and placed my Ranco temp contoller probe in it. Afer it leveled off it read 72 degrees! To sum it up the old style temperature sensor failed. It was the very small skinny model. I went up to Saltycritter and they had the new larger and more solid probe in stock. I took it back home and plugged it in and the water was up to 75 at this point. I think it is time for a seperate indenpendant temperature contoller.