I don't.. only because I don't have a fish room yet. But depending on which kind you buy, many of the new ones have a safety shut off at a certain temp and then kick back on when the temp drops a certain amount. They don't stay on/heating all the time.
Plus the risk of fire decreases even more if you set them up and use them properly. Following the guidelines about where they should be placed, nothing within a certain distance from them, nothing placed over/on them, proper airflow around them to distribute the heat well, etc.
But I have had to use them in my house a couple years ago when my furnace went out and I had to wait almost 2 weeks to have it replaced. They do the job!