I use two smaller heaters in my 40 gallon; that way if one fails the other can work doubletime to keep the tank warm and if it goes bonkers it is not powerful enough to boil my fish (I'm am paranoid)
And I am sooo paranoid because that tank happens to have one of out two most favorite fish..Dutch the baby The Black Ghost Knife
Dutch was very small at the start of the year. He is now over 7 inches.
(I heard they like clean water..I keep it so anyway. I have everyone at a toasty F80 in this house) They fare badly when small if fin nippers are in the tank. They have no back up locomotion if the bottom fin gets ruined. They are much more congenial than their Clown and Brown counterparts (mine is overly
so). They are lurking predators unlike their counterparts..so it is a case of one less fish in the morning.
I hear that they can eat prey that is very large in comparison. Mine refuses anything less than a dainty bit (the betta went into the hidey hole and STOLE the scallop piece right out of his mouth...and kept its fins!)
If you want neat info go to the university experiments about electric fish.