High temps

Looking at the tank today, I noticed that our temp was super high. It was 84.7. It usually stays between 79 and 81. I unplugged the heater just in case it broke and got stuck on. I also pointed a fan at the tank, and put a baggie with ice in the sump to bring the temp down. It's at 80.6 now, so it worked. I'm just wondering why the temp went up so much. I know it's been hot the past couple days, but when it was hot outside previously, it never went up past 82, and that was with the lights on. Our house is sitting at 78, so... again, I'm confused. Any help would be appreciated!

 

Got the temp down to 79 yesterday. Turned the fan off, and took the ice out. Went to look about 20 minutes later, and it said it was back up to 82.6. Stood there for a minute watching it, and it went to 84, 86, got all the way up to 95 in the span of a minute. Went and got my food thermometer, and it read 78.4. So, it's the thermometer. Don't know whether it needs new batteries or is just shot, it's brand new. We got it when we set up the tank about 2 months ago, so I wouldn't think it would need new batteries yet. We'll see.

Also, our heater has no lights on it. We talked to Alex at Aquatic Environments yesterday and that's what he suggested to look at too. We didn't think there were any lights on it, but double checked when we got home and there are in fact no lights on our heater. Weird, I know, but we bought the whole tank used, so whatever heater the people had gotten before us had no lights.