Yeah, those nano cubes do have an unfortunate tendency to overheat and a chiller would be a most worthwhile investment if you would like to keep Zulus (Hippocampus capensis).
There are some very affordable mini aquarium chillers that would be just the ticket for a 12-gallon tank. For example, the CoolWorks Ice Probe and Microchiller units are ideal for small tanks (10-15 gallons) will drop the water temperature up to 6-8°F below the ambient room temperature:
Click here: CoolWorks Ice Probe with Power Supply – Marine Depot – Marine and Reef Aquarium Super Store <http://www.marinedepot.com/md_viewItem.asp?idProduct=CW1111>
Click here: CoolWorks Microchiller – Marine Depot – Marine and Reef Aquarium Super Store
Best of luck getting your nano cube set up to create an ideal environment for Zulu-lulus, curio!