Hydor - Slim Heater
Hydor Slim Heaters are a type of passive heater that warm small volumes of water just a few degrees above the ambient temperature. They do not have a thermostat and are pretty much always on. They work well in tanks of 3 gallons or less if the ambient temperature of the tank is just a handful of degrees lower than your desired temperature.
Rivers to Reef says: We usually do not recommend these types of heaters, but people still like them because you can bury them under your gravel and hide them in smaller tanks. We do not like that they do not have a thermostat, so in warmer temperatures you would need to unplug it so it doesn't overheat the tank.