I use this one.

http://www.amazon.com/gp/product/B000KJDCBA/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1

It's been pretty reliable for me, I guess. It's really easy to read.. haha. As long as both needles are in the yellow zone, you're good! I have a 20 gallon long tank, I put it on the opposite end from my heater. Works great for me and my hermies are happy!