I just bought 4" wide heat tape, 6' long. I bought a ESU Reptile Electronic Controller to regulate it. I have 2 tanks on a 6' shelf, same size, 20 gallon long on it. I have it set for 85 on the warm side and I can't get it past 79. Is 4" wide enough? The guy who sold it to me said it was but I'm questioning his judgement now. My thermometer is one of those double circle humidity and thermometers by Zoo Med that is velcroed on the side of the tank at the bottom...slightly frustrated