An aquarium refrigeration unit, commonly called an aquarium chiller, is a device that actively removes heat from tank water by running it through a refrigerated coil or heat exchanger. If you're keeping cold-water fish like trout, axolotls, or saltwater species from cooler Pacific waters, or if your tank regularly climbs above 80°F during summer, a chiller is the most reliable way to hold temperatures in the required range. Fans and ice packs are stopgap measures. A proper refrigeration unit is the only method that reliably maintains a target temperature regardless of ambient conditions.
This guide covers how aquarium chillers work, the different types available, how to size one correctly, installation options, and how to minimize operating costs since chillers are the most energy-intensive piece of aquarium equipment you'll own.
How Aquarium Refrigeration Works
Aquarium chillers use the same refrigeration cycle as an air conditioner or household refrigerator. A compressor pressurizes refrigerant gas, which then moves to a condenser where it releases heat to the surrounding air. The refrigerant then passes through an expansion valve, which causes it to rapidly cool, and then into an evaporator coil where it absorbs heat from the tank water flowing past. That cooled water returns to the tank and the refrigerant loops back to the compressor.
This cycle removes heat continuously. The chiller's output is measured in BTUs per hour or in fractions of a horsepower (1/10 HP, 1/15 HP, 1/4 HP, etc.). Larger horsepower ratings remove more heat per hour and handle bigger systems.
The incoming water temperature, the target temperature, and the volume of water all determine how much cooling capacity you need. A chiller running constantly at 90 percent output will wear out faster than one running at 60 percent, so proper sizing matters for both performance and longevity.
Types of Aquarium Chillers
Drop-In Coil Chillers
Drop-in chillers use a refrigerated titanium coil that you place directly in your sump or tank. The refrigeration unit sits outside the water and runs the cold coil through a hole or over the tank rim. TetraTec and older Aquanetics models used this approach. Drop-in coils are uncommon in modern retail but persist in aquaculture and large public aquarium installations because they don't require an external water loop.
Inline Flow-Through Chillers
Inline chillers are the most common type for home aquariums. Tank water is pumped out of the sump, through the chiller's heat exchanger, and back. The refrigeration unit sits entirely external to the tank. This is the design used by IceProbe, JBJ, Aqua Euro, and the popular Teco TK series.
The JBJ Arctica 1/10 HP Chiller (model DBA-075) is widely recommended for tanks in the 50 to 75-gallon range. It uses a titanium heat exchanger to resist corrosion, handles saltwater and freshwater, and operates at around 60 to 70 watts in normal use. The Teco TK500 is a step up, rated at 1/7 HP and suitable for up to about 130 gallons.
For large systems, the Aqua Euro USA 1/3 HP chiller handles tanks up to 220 gallons. Commercial operations running multiple large tanks often plumb several chillers in parallel on a shared chilled water loop.
Thermoelectric Chillers
Thermoelectric (Peltier) chillers use a solid-state semiconductor to transfer heat without a compressor or refrigerant. They're quieter and cheaper to buy, but much less efficient. A typical 1/10 HP compressor chiller removes 3 to 4 times more heat per watt of electricity compared to a thermoelectric unit.
Thermoelectric chillers like the IceProbe work for small tanks under 30 gallons or for maintaining temperatures only a few degrees below ambient. If your room is 78°F and you need 72°F water, a thermoelectric unit can manage that. If you need 65°F water in a 78°F room, you need a compressor chiller.
Sizing an Aquarium Refrigeration Unit
Getting sizing right prevents both undersized units that run constantly and oversized units that short-cycle (turn on and off too frequently). Short-cycling wears compressors out faster.
The calculation starts with heat load, which is the total amount of heat your equipment and environment add to the tank water per hour. Equipment like lighting, powerheads, and return pumps all add heat. LED lighting adds less heat than older metal halide fixtures, but high-output LEDs still contribute.
A practical rule: for every 10-degree drop required (say, from 78°F ambient to 68°F target), size for approximately 1 watt of chiller capacity per gallon of water. For a 100-gallon system needing a 10-degree drop, look for at least a 100-watt equivalent chiller, which corresponds roughly to a 1/15 to 1/10 HP unit.
For a 15-degree or greater drop, double that figure. A 100-gallon tank needing 72°F in a room that reaches 82°F in summer needs at least a 1/4 HP chiller.
Always size up by at least one model when in doubt. The difference between a 1/10 HP and 1/4 HP chiller is maybe $150 at purchase but can mean years of additional compressor life.
Installing an Inline Chiller
Inline chillers connect into your existing filtration plumbing. Most connect with flexible vinyl tubing (typically 1/2-inch or 5/8-inch ID). You run a line from your sump's return pump output into the chiller inlet, then from the chiller outlet back to the sump or directly to the display tank.
Flow rate through the chiller matters. Too fast and the water doesn't spend enough time in the heat exchanger to cool adequately. Too slow and the chiller struggles to maintain setpoint. Most manufacturers specify a target flow range, typically 100 to 300 GPH for units in the 1/10 to 1/4 HP range.
Install the chiller with at least 6 inches of clearance on all sides for airflow. Chillers exhaust warm air out the back or top, and restricted airflow raises the condensing temperature, reducing efficiency and stressing the compressor.
Use the chiller in conjunction with an external temperature controller (the Inkbird ITC-308 is a popular choice) for more precise control, or rely on the chiller's built-in thermostat if it's well calibrated. Most modern inline chillers have digital thermostats accurate to within 1 to 2°F.
For an overview of cooling equipment alongside other essential hardware, browse our Best Aquarium Equipment roundup.
Running Costs and Efficiency Tips
Chillers are power-hungry. A 1/10 HP compressor chiller draws approximately 60 to 90 watts when running. Running 8 hours per day, that's 480 to 720 watt-hours, or about $2 to $4 per month at average US electricity rates. A 1/2 HP unit running 10 hours daily in summer costs $8 to $12 per month.
To reduce operating costs: - Keep the room cooler. Every 5°F reduction in room temperature cuts chiller runtime by 20 to 30 percent. - Minimize heat-generating equipment. Replace metal halide fixtures with LED equivalents. Upgrade powerheads to efficient DC models. - Insulate the tank sides. Foam board insulation on sump walls and return plumbing reduces heat gain. - Run the chiller on a timer during the hottest part of the day if your controller allows it.
For additional equipment recommendations that complement your cooling setup, check our Top Aquarium Equipment guide.
FAQ
What temperature can an aquarium chiller maintain? Most inline compressor chillers can cool water 15 to 25°F below the ambient room temperature. For extreme cooling (holding a 55°F coldwater tank in a 75°F room), you need a full 1/2 HP unit or a commercial-grade chiller.
How noisy is an aquarium chiller? Compressor chillers produce a humming noise similar to a small window air conditioner, roughly 45 to 55 dB when running. This is noticeable in a quiet room. Placing the chiller on a rubber anti-vibration pad and positioning it away from seating areas helps. Thermoelectric chillers are near-silent but much less powerful.
Can I use a chiller on a freshwater tank? Yes. All inline chillers work with freshwater. If you're keeping axolotls (which need 60 to 68°F), or a coldwater biotope with species from mountain streams, a chiller is the right tool. Most units are also rated for both freshwater and saltwater use.
Do I need a separate pump to run the chiller? Not always. If your existing return pump has sufficient output after supplying the display tank, you can tee off a line to the chiller. Alternatively, use a dedicated small circulation pump feeding the chiller independently. A dedicated pump gives you better flow control since you can dial it separately from your return rate.