Why aquariums overheat
Tanks gain heat from the room through the glass and water surface, and from equipment: submersible pumps and powerheads put nearly all their electricity into the water, external pumps a good part of it, and lights add heat from above. Reef corals and many fish are stressed above about 82 – 84 °F (28 – 29 °C), and cold-water species like axolotls need much cooler water. When room temperatures climb in summer, a chiller keeps the tank stable.
Heat gain (BTU/h) ≈ tank surface area × ΔT + equipment watts × 3.412 (share entering the water)
Typical chiller sizes
| Chiller | Approx. capacity | Typical tank size (ΔT ≈ 5 – 8 °F) |
|---|
| 1/10 HP | ≈ 1,100 BTU/h | up to 30 – 60 gal |
| 1/6 HP | ≈ 1,800 BTU/h | 40 – 100 gal |
| 1/4 HP | ≈ 2,600 BTU/h | 80 – 180 gal |
| 1/3 HP | ≈ 3,500 BTU/h | 120 – 250 gal |
| 1/2 HP | ≈ 5,200 BTU/h | 200 – 400 gal |
| 1 HP | ≈ 10,000 BTU/h | 400 – 800 gal |
Capacity ratings vary between manufacturers and with room and water temperatures – check the performance chart for your conditions.
Cheaper ways to keep a tank cool
- Evaporative fans blowing across the surface can drop the temperature 2 – 4 °F; top off with RO/DI water.
- LED lighting runs much cooler than metal halide or T5 fixtures.
- DC pumps that throttle down add less heat than oversized AC pumps.
- Cool the room – air conditioning set a few degrees lower helps every tank in the room.
- Lights on at night, when the house is cooler, shifts heat away from the hottest hours.
Installing a chiller
Most aquarium chillers are plumbed inline after the return pump or fed by a dedicated pump from the sump. Match the flow rate to the manufacturer's recommendation – too little flow causes icing, too much reduces efficiency. Give the chiller plenty of air around its condenser and keep it out of closed cabinets. Use a separate temperature controller or the built-in thermostat with a small differential to avoid frequent cycling.
Frequently asked questions
What size chiller do I need for a 75 gallon tank?
Usually a 1/10 – 1/6 HP chiller, depending on how much warmer the room gets than the tank and how much equipment heat there is.
How many BTU is a 1/4 HP aquarium chiller?
Roughly 2,000 – 3,000 BTU/h depending on the model and conditions.
Can fans replace an aquarium chiller?
For small temperature differences (2 – 4 °F) evaporative fans often work; larger differences need a chiller.
What temperature is too hot for a reef tank?
Sustained temperatures above about 82 – 84 °F stress most corals.
Do I need a chiller for an axolotl tank?
Often yes in warm climates – axolotls prefer about 60 – 64 °F, well below typical room temperature.
About TankChill
TankChill is an independent, free sizing calculator for aquarium chillers on reef, marine, freshwater and cold-water tanks. Results are estimates; chiller capacity varies with conditions, so check the manufacturer's charts. We are not affiliated with any aquarium equipment brand.
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Last updated: October 28, 2026
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