When was the last time you checked or replaced your heaters? if the answer was never, now is a good time to do so. During the summer months where in some parts of the world can get up to over 100F degrees, we are looking at getting chillers and trying to keep our tanks cool in hopes that we don't over heat our critters and corals.
As we start getting into the cold weather months, heaters start getting taxed to keep your tank up to temp. And this is when you can see a failure happen, when your tank doesn't keep its temp. It's always good practice to keep spare equipment when it comes to any hobby, and heaters are no exception ours. Always keep at least 1-2 spare heater on hand in case your heater fails. Also, if you are not using a external controller to monitor/control your heater and tank temp it might be a good time to invest in one as the built in thermostats are not that reliable.
Also remember that the recommended wattage is 3 watts per gallon. So for a 100g tank you'll want to have at least 300w of heating power. Its also best practice to use smaller heaters to heat your tank. For the same 100g tank, you can use 3 100w heaters, or 2 150w heaters for redundancy.
Good idea! I always check them in the winter and replace every 2-3 years.