You turn the a/c off when you leave? If that is the case, we found the cause of your dead shrimp. When you leave and turn the A/C off I'd bet that the temperature of your water would have no problem swinging upwards of 4 degress very quickly!
Coralife makes a cheapy 7 dollar digital thermometer that you should look into getting. I'm sure Carlos has them at RCA or can get one.
John