I found that tubs in cooler than 75 could not hold ambient air temps warm enough. After a lot of messing about I abandoned them for this reason and an using a insulated rack custom enclosures and tanks.

One thought might be (expensive one but it will solve the problem) is to go ahead and buy a adult enclosure (vision or the like) and place the tub inside it. if you have a fluorescent light (tube style I use 2 footers) inside the adult enclosure it will go a great length to maintaining the ambient air temps warm enough to manage a cool room. My adult enclosures hold ambient temps about 10-15ºF over the room temps.

It is one idea anyway.