I need a good option for heating my tank that isn't too expensive. I have a 120g cube w/ a 100 gallon sump that probably holds 60-80 g of water at any given time. I have three standard heaters in there now the biggest being a 400w heater. During the summer all was good, but now that it is getting colder I have noticed my temp isn't staying consistent. If I leave the thermos set to 79 the tank slips to 74, but if I turn them up to 80-83 to compensate the take gets a little too hot. Just seems like there struggling now.

I don't know much about controllers and probes so any info would be appreciated.

Thanks