12v DC powered fans, varied speed based on ambient air temperature. Sensor should not be submerged in water.

To regulate temperature of a reef tank, installing cooling fans is the least expensive solution.

Fans can be used in the canopy area to blow light-generated heat away from the water, or inserted into a fan tray bracket to blow down on the surface of the water within the sump.

These fans have a thermistor sensor that will increase the speed of the fan based on ambient air temperature readings. Running at the lowest speed will still do an excellent job and do so quietly.  These fans have an excellent reputation for longevity, which is why they are the only fans I use for my reefs for the past 10 years.

The 12v DC power supply can be plugged into a timer or controller to specify when you want the fan to turn on and off.  For my reef, I've programmed the Apex controller to turn the fan on when the tank temperature rises above 79.5°F, and to shut the fan off when the temperature drops to 79°F.  The 80mm or the 120mm fan is the perfect accessory for inexpensive evaporative cooling.  Use it to force cool air into a canopy or stand to drive warm air out, or installed in an acrylic fan tray mounted over an area of the sump to cool the water.  Use this fan with a timer or a controller to dial in that perfect schedule to regulate the water's temperature.  

If you need an acrylic fan tray to install these over your sump, I build them: https://www.melevsreef.com/reefshop/acrylic-products/brackets/fan-tray-bracket (be sure to specify fan size)

80mm / 3" fan
120mm / 4" fan
Variable speed based on thermistance
12v DC power supply