I put the probe under the carpet because I didn't want him to get tangled on the wire running to it or to move the probe around. I figured the carpet is only maybe a 1/8th of a inch thick, temps shouldn't be but a couple degrees difference I wouldn't think. I went ahead and ordered the Herpstat 2 so I can properly regulate the two areas to the temperatures I want. Do any of you strictly use UTH's to heat your tanks?