I finally found what I believe is the problem. The new thermometer I purchased is running about 10 degrees off - too low. I just assumed it was accurate. From now on, I'm always using 2 thermometers so this never happens again.
I discovered the problem when I talked to the hatchery and they suggested I get them into the most temperature stable environment I have which is inside the house. I put a thermometer in there and just tossed the "new" one in there rather than put it away. I found this by accident and I'm glad I didn't keep using the bad one or more would die.
Thanks to everyone for your help. I really appreciate it.