"Make sure they work" is excellent advice. Just because you see a light on or a heater controller displays that it's working does not mean the heater element is in good shape. A titanium heater w/heater controller looked like it was working last fall on my FRT's tank. The water was actually 74F due to the heater's core elements failing. Almost lost my FRT to hypothermia. It's a good thing I buy my heaters by the case to ensure I have ready-to-use spares on hand.
Keep a spare heater on hand for emergency replacement. The cost of a spare is far outweighed by the investment you have in your fish.