You don't want the probe in a tub. You want it directly up against the heat tape so that its directly measuring the heat coming off of the tape. Do you have one solid piece of flexwatt or multiples?

Youre going to have fluctuations in temps between tubs on the top and bottom. When I had a melamine rack my middle tub was at around 89, four rows up was 93, bottom tub was around 87.