What's the best or most reliable under tank heater for a tub set up?
Most people that have tub set ups or racks use 3 inch flex watt heater tape, you can purchase it from Bean Farm, LLL Reptiles or Big Apple. You will also need a proportional thermostat, helix makes a good one so does herp stat from robotics.
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I like the heat tape to cover about 1/4 of the bottom of the tub. It comes in a variety of width's, and you can cut it or get it cut to whatever length you need from where-ever you purchase it from. Personally, I think you're better off going with a heat tape like Brian mentioned above than you are going with a "mat" style that you'll get in a pet store. A mat may run you $30, and an equally good, accurately sized heat tape will be maybe $5.