My hydrofarm doesn't fluctuate nearly so badly, but I have both girls' tubs on top of a dresser rn, not in an enclosed rack. Lowest read I've gotten in the off position is 87 and the highest was 92. This is with it set at 95f. I will def switch to a herpstat with proportional regulation when I build my rack though. the way the racks are set up they're gonna tap sort heat better than what a tub by itself will.