I keep my hedgie in a tub too. I keep him in my office which means he has his own room. I have a thermometer in the room next to his tub which I monitor, if I feel the temperature dropping and the thermometer confirms it, I close his door and put his heater on. He has an oil heater with built in thermostat so the room doesn't overheat.
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"Bramble: a prickly scrambling shrub of the rose family, especially a blackberry."