I have an XPVC rack that overall works very well. The heat tape reads 90, and that's the temp in my Vision tubs. However, I have a couple young ones in Sterilite 6 quarts, and the temp inside the tub was reading 70 degrees despite the heat tape being 90! Is it normal for Sterilites to not warm up?

I just now tried taping styrofoam to the back and back sides of an empty tub to see if that helps, but it's kind of an inelegant solution.