Not sure what type of temperature deltas you are facing between tank and room temperature, but I used this wee tiny digitally set thermometer for my 7.5 gallon cube as it was one of the few to fit in the chamber of my Tunze 3161 filter: Archaea Mini Heater (25W). I would assume a Waterbox gives you a little bit more room, so maybe a Cobalt Neotherm heater?