What heater should I use in a 29 gallon biocube?

I bought a 150 watt marineland visi-therm heater. It is too big and bulky and seems to heat the water more than I want it to. For instance, if the dial is on 78, it will raise the water temp to 81. I want to be able to hide the heater, but this one is too big. Is there any way I can hide the heater on the right chamber by the filter? If so, what heater would fit? Any suggestions would help.
 
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