Reef tank temperature: ideal range, safe swings, heaters, and chillers

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Temperature is the reef tank parameter you never have to dose, and the one that can wreck a tank fastest when equipment fails. Corals tolerate a fairly wide range of temperatures, but they do not tolerate sudden or sustained departures from what they are used to. This guide covers what temperature to keep, how much swing is reasonable, what heat does to oxygen and coral bleaching risk, and how to set up heating and cooling so a single failure cannot cook or chill your corals.

What temperature to keep a reef tank at

Randy Holmes-Farley, a chemist and longtime Reefkeeping Magazine columnist, recommends 76 to 83 °F "unless there is a very clear reason to keep it outside that range." He keeps his own tank at about 80 to 81 °F. He also notes that the greatest variety of corals in the wild grows in water averaging about 83 to 86 °F, so why not run that warm? Because a closed tank has less margin. Warmer water holds less oxygen and speeds up every animal's metabolism, so during a power failure a warm tank runs out of oxygen sooner. His practical tweak is to run a little cooler in summer, when an outage would let the tank warm up, and a little warmer in winter, when it would cool down.

Within that range, the exact number matters much less than holding it. Corals acclimate to the temperature they live at, and what stresses them is change: a heater that sticks on, a summer week without cooling, or a cold snap during a power cut.

Why stability matters: the bleaching threshold

The best data on temperature stress in corals comes from wild reefs. NOAA Coral Reef Watch defines a reef's "normal summer" as its Maximum Monthly Mean (MMM), the long-term average temperature of the warmest month. Following a lab study by Glynn and D'Croz (1990), it treats 1 °C above the MMM as the bleaching threshold. Heat stress then adds up over time as Degree Heating Weeks (DHW): a week spent 2 °C above normal counts as 2 °C-weeks, and so does two weeks at 1 °C above.

Aquarium corals are not a wild reef, and nobody has measured an equivalent scale for tanks. The lesson still carries over: your setpoint is your tank's "normal summer." A sustained rise of about 1 °C (1.8 °F) above it, especially one that lasts days or weeks, is the kind of exposure that bleaches corals in the ocean. High light makes heat stress worse, which is why our coral bleaching science guide recommends dimming lights during a heat event. Corals that brown out or pale after a hot spell are telling you the same thing.

How much daily swing is OK?

Some reefs swing a lot. Oliver and Palumbi (2011) studied two back-reef pools in American Samoa: one ranged about 26.5–33.3 °C, while a more variable pool ranged about 25.0–35 °C, with 2–3 hour heat spikes during summer low tides. When they heat-stressed the table coral Acropora hyacinthus from both pools, corals from the moderate pool showed nearly 50% mortality, while corals from the variable pool showed only about 17%, not statistically different from unheated controls. The authors conclude that living with regular fluctuation raised the corals' heat tolerance.

That is not a reason to let your tank wander. Those corals met predictable, tide-driven spikes every summer and acclimatized to them over a lifetime. A tank's swings usually come from equipment cycling or failing, and they are neither predictable nor something the corals were raised on. For a home reef we suggest:

• Keep the day-to-night swing to about 2 °F (1 °C) or less. Lights, pumps, and room temperature usually push the tank warmest late in the light period and coolest just before dawn.
• Make any planned change slowly, about 1 °F per day at most if you are moving the setpoint.
• Log temperature with every test on our printable reef test log, or better, run a controller that records it continuously. A swing that widens week by week often means a heater or chiller is starting to fail.

Temperature and dissolved oxygen

Oxygen is less soluble in warm water. The table uses the Garcia and Gordon (1992) equations for full-strength seawater (salinity 35) in equilibrium with air at sea level, which is the most a well-aerated tank can hold.

Going from 24 to 30 °C costs about 9% of the oxygen the water can hold, while fish, corals, and bacteria all use oxygen faster in warmer water. That squeeze is why overheating and power outages are so dangerous together, and why strong surface agitation and a running protein skimmer matter most in summer. During an outage, a battery air pump is the cheapest insurance you can buy.

How temperature affects the rest of your tank chemistry

• Alkalinity and calcium use. Warmer tanks generally run faster metabolisms, so alkalinity and calcium consumption can rise in summer and fall in winter. If your alk drifts with the seasons, check the dose with the dosing calculator and see why alkalinity swings.
• Evaporation. Warm water and fans evaporate more fresh water, so your auto top-off runs more. If you dose kalkwasser through the top-off, the kalk dose rises and falls with it (see kalk and evaporation).
• Salinity readings. Specific gravity depends on temperature, and refractometers with automatic temperature compensation still read best with samples near room temperature. See salinity calibration and convert between ppt and SG with the salinity converter.
• pH. pH probes and test kits are temperature sensitive, and tank pH swings daily with photosynthesis. Read pH at a consistent time and temperature (see reef tank pH).

Heaters: sizing, redundancy, and the stuck-on failure

Most heater failures fall into two kinds: the heater dies and the tank slowly cools, or the thermostat sticks on and the heater keeps heating. The second is the one that kills tanks, and the math shows why it can happen overnight.

Heating math: seawater needs about 4.0 kJ per kg per °C. 100 US gallons of seawater (about 387 kg) therefore needs about 1.55 MJ, or 430 watt-hours, to warm by 1 °C. A 300 W heater supplies that in about 1.4 hours if no heat escapes. Most of a heater's working time goes to replacing heat lost to the room, which is why a tank in a cold room needs more wattage than the same tank in a warm one.

Now run that in reverse. The table shows how fast a heater that sticks fully on warms the water, ignoring heat lost to the room. Real tanks lose some heat, so the actual rise is slower, but in a warm room it gets close to these numbers.

An oversized heater on a small tank can push the water several degrees past the bleaching threshold within a few hours. The fixes are simple:

1. Use an external temperature controller with its own probe, and set the heater's built-in thermostat a degree or two above the controller's setpoint so the controller does the switching and the heater's thermostat is the backup. Pick a controller with a high-temperature alarm or cutoff.
2. Split the wattage. Two heaters that together cover your need are safer than one large heater: if one sticks on, it adds only half the heat, and if one dies, the other keeps the tank from crashing cold.
3. Do not oversize. Follow the heater maker's sizing chart for your tank volume and the coldest your room gets, rather than buying the biggest heater on the shelf.
4. Put heaters in flow, usually in the sump, and keep them fully submerged at the lowest water level your sump reaches. Unplug them before a water change drops the water level.
5. Check the controller against a second thermometer every few weeks. Probes drift, and a controller that reads 2 °F low will quietly keep the tank 2 °F too warm.

Cooling: fans, lights, and chillers

Pumps, lights, and a warm room all add heat. In summer, many tanks need active cooling to stay at their setpoint.

Evaporative cooling math: evaporating water at 25 °C absorbs about 2.44 MJ per kg. One US gallon (3.785 kg) evaporated per day removes about 9.2 MJ, which averages out to about 107 watts of continuous cooling. A fan that raises evaporation from 0.5 to 1.5 gallons per day adds roughly 107 W of cooling, at the cost of a gallon a day of extra top-off.

• Fans blowing across the sump or display surface are the cheapest cooling. Put the fan on the temperature controller so it runs only when needed, and make sure your top-off reservoir can keep up.
• Lights. Shorten the photoperiod or dim the fixture during heat waves. Less heat goes in, and lower light also eases heat stress on the corals' algae.
• Room cooling. Air conditioning or simply keeping the tank room shaded and cooler helps every other method.
• Chillers remove heat directly and are the answer when fans and lights are not enough, in hot climates, or in tanks with lots of pumps and high-output lighting. They need good airflow around them and add their own heat to the room.

Power outages

In an outage, the problem is oxygen first and temperature second. Keep water moving and the surface broken with a battery air pump or a battery-backed return or powerhead. In winter, wrap the tank with blankets to slow heat loss. In summer, float sealed bags of ice (never loose ice) to slow warming, and keep lights off. Holmes-Farley's seasonal setpoint advice (cooler in summer, warmer in winter) buys extra time in either case.

Temperature gear we would start with

We list these by name only for now. Check current reviews and your tank's needs before buying.

• Controller with alarm or cutoff: Inkbird ITC-308, a widely used dual-stage plug-in controller (one outlet for heating, one for a fan or chiller) with a high- and low-temperature alarm.
• Heaters: Eheim Jäger and Cobalt Aquatics Neo-Therm, two long-running heater lines. Buy two smaller units rather than one large one.
• Backup thermometer: any accurate stand-alone digital or glass thermometer to cross-check your controller's probe.

Worked example

A 75-gallon reef runs one 300 W heater, set to 78 °F, with no external controller. One night the thermostat sticks on. With almost no losses the water warms about 1.7 °F per hour, so it passes 80 °F in a little over an hour and 86 °F in under 5 hours, around 4.4 °C above the setpoint by morning. The water's oxygen capacity has dropped about 7% while every animal is burning oxygen faster.

The same tank with two 150 W heaters on an external controller set to 78 °F, with each heater's dial set to 80 °F and an 81 °F high alarm: if one heater sticks on, the water warms at about half the rate. The controller cuts power to both heaters once the probe reads above its setpoint, and the alarm sounds at 81 °F if something else goes wrong. The worst case is a degree or two, and you hear about it.

Common temperature mistakes

• Relying on the heater's own thermostat as the only control.
• One large heater sized "to be safe" on a small tank.
• Chasing a "perfect" number instead of holding one steady setpoint.
• No cooling plan until the first summer heat wave.
• Never checking the controller probe against a second thermometer.
• Changing the setpoint by several degrees in one day.

FAQ

What temperature should a reef tank be? Chemist Randy Holmes-Farley recommends 76 to 83 °F (about 24.4 to 28.3 °C) unless there is a clear reason to go outside that range, and keeps his own tank at about 80 to 81 °F. Pick one setpoint inside that range and hold it steady; stability matters more than the exact number.

How much daily temperature swing is OK in a reef tank? There is no single proven limit. We suggest keeping the day-to-night swing to about 2 °F (1 °C) or less. A tight band is easy to hold with a controller, and it makes a failing heater or chiller stand out quickly in your log.

What temperature causes coral bleaching? On wild reefs, NOAA Coral Reef Watch counts heat stress once water sits at least 1 °C (1.8 °F) above the local average for the warmest month. About 4 °C-weeks of accumulated stress brings significant bleaching, and about 8 °C-weeks brings severe bleaching and mortality. In a tank, treat your normal setpoint as that baseline and act on any sustained rise of 1 °C or more.

Should I use one big heater or two smaller ones? Two smaller heaters that together cover your need, run through an external temperature controller with a high-temperature cutoff, are safer. If one heater sticks on, it adds only half the heat, and the controller can cut power before the tank overheats.

How do I cool a reef tank in summer? Blow a fan across the sump or display surface, since evaporating water carries heat away (about 107 W for every US gallon evaporated per day). Shorten or dim the lights, keep the room cooler, and use a chiller if those are not enough. Evaporation raises top-off use, so watch salinity and any kalkwasser dosing.

References

• Holmes-Farley, R. (2004). Reef Aquarium Water Parameters. Reefkeeping Magazine, May 2004. Temperature recommendation 76–83 °F; seasonal setpoint reasoning.
• NOAA Coral Reef Watch. Methodology of the Version 3.1 daily global 5km coral bleaching heat stress products. MMM, HotSpot, Degree Heating Week, and Bleaching Alert Area definitions (alert levels updated December 2023).
• Glynn, P.W. & D'Croz, L. (1990). Experimental evidence for high temperature stress as the cause of El Niño-coincident coral mortality. Coral Reefs 8: 181–191. doi:10.1007/BF00265009
• Oliver, T.A. & Palumbi, S.R. (2011). Do fluctuating temperature environments elevate coral thermal tolerance? Coral Reefs 30: 429–440. doi:10.1007/s00338-011-0721-y
• Garcia, H.E. & Gordon, L.I. (1992). Oxygen solubility in seawater: Better fitting equations. Limnology and Oceanography 37(6): 1307–1312. doi:10.4319/lo.1992.37.6.1307. Table values calculated by Coral Compass from the paper's equations (salinity 35, 1 atm).