I've never used a hydrofarm, so I guess I'm not really sure if that'll work or not. :/ If it is mean to be used in soil then you're probably okay, but somebody might come along and correct me. Just make sure when you hot glue down the probe you don't glue over the actual probe part, it'll make the heat reading part freak out and could give you imprecise readings. I would just glue down the cord near the probe in that case.

If you want to put it between the UTH and the bottom of the tank just peel it away, put your probe into place on the heat mat, then carefully stick the heat mat back down. So long as there's some kind of airflow for the UTH already in place then you'll be fine.