If been trying to use a submersible heater to keep my pyxie's water pool (dimensions 13" long, 7" wide) warm on the cooler side. It's one of those from walmart that only goes up to 78 degrees. I have a stealth heater that can get warmer than that but it gets hot to the touch and I'm afraid she would get burnt by it. Are there any heaters with adjustable knobs that don't pose any burn risks at all, or is the mid/high 70s fine for water temps? The warm side, with the substrate, is kept in the constant mid 80s via infrared heat lamp.