in the winter fish can handle lower temps like 72-76. I personally keep my tanks that have South American cichlids and Central American cichlids at 74 degrees in the winter. It helps keep my electric bill down and i know they can handle it and do very well in cooler temps. If you can keep your tanks at least 74 your fish will be fine, the whole " you need to keep fish at 78 degrees" is absolutely not true because they can and sometimes prefer to be in cooler temps. I know Cory has made a video saying that most fish actualy don't need heaters in their tanks. It all depends on the species you are keeping.
