Did you test the water before or after the water change?
I don't think your heater not being on for a couple of hours is what did it unless your tank is in an area that's 60 degrees, even then I don't think it would have dropped that much in only 2 hours. My heater doesn't even run normally unless its the middle of the night. We don't have central air and I don't run my heater in the living room. But how cold was the water change water? I guess that could have dropped your temp if you changed 25%.
Do you not have a controller?
As far as test kits, I like the Hanna Alk Checker... but have also used the Salifert. Others really like the Red Sea Alk test kit.
I use Salifert for Cal and Mag.
