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Call me old fashioned, but I do not like digital themometers. They don't last as long as the old style glass thermometers (I have some glass models that are over 20 years old) and are no more accurate.
I've for the last few years used cheap indoor/outdoor thermometers, the kind with one sensor at the end of a long cable - something similar to this: http://www.brannan.co.uk/images/pl/13-420-3.jpg
I was a bit worried that the metal tip of the sensor migth corrode, so I smeared silicone glue over the sensor and, for good measure, the part of the cable that goes into the tank.
It's worked well.
The reason I use these is because they not only indicate current temperature, but also minimum/maximum temperatures over a time period.