Im sorry if I missed this, but do you have a temp gun at all? What is the exact reading on the heat tape, not the air above it? Even in a 70 deg room the heat tape itself should be able to heat up to a 90 degree hot spot, but a 3 or 4" piece wont do much for the ambients. Taking hot spot temps with a digital probe thermometer can be misleading, imo those are best for ambient temps.
This issue is why I avoid racks with small heat tape. 11" tape works much better for larger sized tubs.
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