First, I have the little temp gun you have, and a much more expensive automotive one. They both read the same. Second, put the heater under, not inside, the tank. If the probe is big, just stick it under the heater, rather than wedged between the heater and the glass. Also, if you are using under tank heat, you will want to keep substrate thin so the snake is exposed to the heat, though they will push some substrate aside to reach the heat.
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