You need a digital thermometer with probe, a very reliable one is the acurite 00891 for $12 on amazon. You could also get a reliable temp gun but they cosy more than the acurite. You put the thermostat probe sandwiched between the uth and bottom of tank then you put the thermometer probe under the substrate right against glass over the uth and set the thermostat so the thermometer never reads hotter than 94.

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