Where are you located? I've had success brumating crotes at temps in the low 60s. No heat is necessary as long as you don't have a draft and the temps don't fall below the high 50s. Everything about these animals slows down at those temps and that's ok. Those are temps that they brumate at in the wild. Again, just check the water on a weekly basis. Because the animal's metabolism is slowed to a crawl, it really shouldn't drop any weight.

-Kris