heating tubs NOT in a rack...?

[Vypyrz]

so you just stick the uth on the bottom of the plastic tub? Do you notice the tub melting?

Just as Pit said, you will need a thermostat to control the UTH. My UTH's are self-adhesive, but instead of permanently sticking them on, I just tape them on with aluminum tape. That way it is easy to remove and re-apply the UTH when cleaning or changing tubs, and you won't have to destroy the heater to remove it.