If your aquarium is like mine during this time of year, then your water temperature will have already started to plummet and those heaters just don’t seem to work. They are on, but their impact is minimal because of their size when compared to total volume of water it has to heat up. To get the most out of your heater setup and to keep the water temps stable, be sure to use multiple small heaters placed in areas of high water flow. Or, if you’re cold too, just crank up the heat in the house. That’ will help keep the aquarium warm too.