No that is how they work. It is an on and off stat delivering full power (causing high temp spikes) and no power periods where it cools off.

I would suggest the combination to be a terrible idea. A tub heats very quickly and cools very quickly unlike something like a glass tank that heats slower and cools slower so it regulates the hills and valleys. The rack does not do this at all. Add THG tape that has a higher temp out put than most tapes this increases the spikes and dies even further.

The hydrofarms also have a lot of hysteresis (about 5 degrees) this also increases the range of the spikes. Together it is a dreadful combination.

I would suggest only a proportional stat be used with a high heat element and a tub. Nothing else will accurately regulate the temp with out issue. Personally given the high heat capabilities of the THG tape I would only use a herpstat with safety relay and keep the hydrofarm as a fail safe for it in case the probe is pulled free.