I am thinking about growing a wooly sundew in a terrarium, and I have everything worked out but the humidity and heat it requires. In Peter D'Amato's book, it mentions using a fish tank heater submerged in a mason jar full of water. We have a heater and fish tank from when my little sisters tried (unsuccessfully) to have goldfish, but the heater says that it should ONLY be used in tanks holding >2 gal. but <5. Now I know that a mason jar holds waaaaayyy less than 2 gallons, but Peter seems to know his stuff. Is the warning just so that you wouldn't overheat your tank?
If I'm understanding this correctly, the heater would be in the mason jar full of water, sitting inside the fish tank terrarium. This should be just fine. You just need to monitor the water temperature, and make sure the jar never goes low on water. Having said this however, you may not need to do this. If your house stays in the 70's it will probably be warm enough for them. Also, if you're using fluorescent lights over the tank, this warms it up quite a bit also. The important thing is to get a thermometer in there so you actually know what the temperature is.