A device that can float in a liquid can be used to adjust the temperature of the liquid by monitoring the temperature of the liquid and comparing to a desired temperature and pumping liquid into contact with a thermal adjustment element in the device. The device comprises a thermal adjustment element, a temperature detector to measure the temperature of the liquid exterior to the device, a controller to turn the device on when temperature measured is outside a desired range, a pump to take a portion of the liquid from outside the device to thermal contact with the thermal adjustment element and return the portion of the liquid to outside the device, and a photovoltaic module mounted in a position to be exposed to light which photovoltaic module provides energy to operate the device.