What temperature should a reef tank be (and is 82 too hot)?
It is the hottest week of the year, the room is 84 F, and the tank that sat at 78 all winter is reading 82 by mid afternoon. The forum thread you found has one person saying 82 is fine because the Red Sea is warmer, and another saying anything above 80 is cooking your corals.
Short version: keep the reef tank temperature between 76 and 80 F (24.5 to 26.5 C), aim for 78 F (25.6 C), and care more about holding it steady than about the exact number. 82 F (27.8 C) will not kill anything this afternoon. It is a place to pass through in summer, not a place to live.
The ideal reef tank temperature
Every reef aquarium temperature chart you will find gives a band close to 76 to 80 F (24.5 to 26.5 C). 78 F is where most people land because it leaves room on both sides: a couple of degrees of cool night before you are out of the band, and a couple of degrees of summer before you are anywhere dangerous.
Randy Holmes-Farley's parameter table, the one most target charts descend from, allows 76 to 83 F (24.5 to 28.3 C). That top end is wider on purpose. 83 F is not harmful in itself. It is just that a tank set to 83 has no margin left, and margin is the whole reason to pick a lower number.
The reef tank parameters pillar has every target in one place and the order to fix them in. Temperature sits near the top of that order because everything else runs at whatever speed temperature sets.
Natural reefs run warmer, so why not copy them
Real reefs mostly sit between 79 and 84 F (26 to 29 C), and plenty spend their summers at 84 to 86 F (29 to 30 C).
But the ocean is enormous, so its temperature moves over weeks, not hours. The corals in a hot lagoon have been growing there for years, adapted to that history. Corals bleach when water climbs about 1 C (roughly 2 F) above the local summer maximum and stays there. What hurts them is not a number, it is a departure from what they are used to. Your tank is 300 litres (80 gallons) in a room heated by the sun, and it can move 5 F in an afternoon. That is the part to worry about.
Is 82 too hot for a reef tank
Plainly: 82 F (27.8 C) is survivable, and most tanks will see it at some point every summer without losing a thing. A reef tank too hot for one afternoon, dropping back to 79 overnight, is nothing to panic about.
What 82 is not is a good set point, for two reasons.
Oxygen. Warm water holds less of it. Seawater at 35 ppt (1.026 SG) saturates at about 6.7 mg/L at 77 F (25 C) and about 6.1 mg/L at 86 F (30 C). Everything alive in the tank respires faster as it warms, and the gap is widest at night, when corals and algae stop producing oxygen. A tank that is fine at 82 F with the lights on can have fish gasping at the surface at 3am.
Margin. From 78 F you have 6 degrees before 84 F (29 C), where sensitive SPS start to go wrong, and 8 before 86 F (30 C), where losses become likely across the board. From 82 you have 2 and 4. A stuck heater or a hot Sunday eats that in hours.
So: 82 for an afternoon, do nothing dramatic. 82 every afternoon for three months, apply one of the fixes below, because you are running with the safety margin already spent.
A reef tank temperature swing versus a steady high
This is the distinction most people miss, and it changes what you should fix.
A tank held at a steady 81 F (27.2 C) all summer is a tank running a little warm. Its inhabitants acclimatise within days. A tank bouncing from 77 F at dawn to 82 F at 6pm and back, every day, is under a different kind of stress, even though its average is lower. The swing never lets anything settle.
The usual rule is a swing of no more than 1 to 2 F (0.5 to 1 C) across a day. Every tank does that much. A swing of 4 F (2 C) or more, every day, is where corals start to show it: polyps retracted in the late afternoon, SPS tips going pale, an LPS that never fully inflates.
So take the temperature at 6am and again at 6pm. If those numbers are 4 F apart, you have a swing, and the fix is flattening the curve. If they are 81 and 82, you have a steady high, and the fix is bringing the whole curve down a couple of degrees.
Where the summer heat comes from
Three sources besides the weather, and the shape of the daily curve usually tells you which one it is.
The room. A tank cannot stay cooler than the room it sits in without a chiller or evaporation doing work. If the room hits 84 F (29 C), the water is heading for 84 F, and turning the heater down changes nothing.
The lights. Metal halides and T5s put real heat into the water, several degrees in a closed canopy. LEDs put in much less, but still show up as a bump that peaks near the end of the photoperiod.
The pumps. Every watt a return pump and powerheads draw ends up in the water as heat. An older AC return pump drawing 100 W around the clock is a 100 W heater you cannot switch off. A tank that stays warm all night, lights off and room cooled, is being warmed by its pumps.
Fix one: fans on the sump
Start here. It is cheap, it is fast, and for most tanks it is enough.
A fan blowing across the water surface in the sump, or across the display if there is no sump, cools by evaporation. A clip fan or a pair of PC fans on a lid will drop a tank 2 to 4 F (1 to 2 C), less in a humid room. A fan pointed at the glass does nothing, and a fan blowing into a closed cabinet just recirculates hot air, so give it somewhere to push the air out.
The side effect is evaporation. A tank losing 1 to 2 percent of its volume a day in winter can lose double that with a fan on it. Evaporation takes water and leaves the salt behind, so salinity climbs. With an auto top off, the cost is refilling the reservoir twice as often. Without one, you top up with fresh RO/DI every day, and if salinity has already crept up to 1.028 SG (37.5 ppt) you bring it back down no faster than about 0.001 SG per day, because a fast salinity move is harder on corals than the heat was. What is the ideal salinity for a reef tank covers where to hold it, and which salinity tester you can trust explains why to check your refractometer against calibration fluid before believing it.
Fix two: the lighting schedule
If the peak of your temperature curve sits on the end of your photoperiod, shorten the photoperiod or move it. Cutting a 10 hour photoperiod to 8 for the duration of a heatwave costs your corals nothing they will notice, and takes the top off the afternoon spike. Shifting the whole schedule later, so peak intensity lands after the room has started to cool, does the same job without cutting hours. Both are free and reversible the day the weather breaks. Do not turn the lights off for days, though. A week of darkness is its own stress.
Fix three: a chiller, and when it is worth it
A chiller is the only fix that works regardless of the room. It is also the most expensive, the loudest, and it dumps every degree it removes from the tank straight back into the room, which is why a chiller in a closed cabinet under the tank often achieves very little.
It is worth buying when fans and the lighting schedule cannot hold the band: a room that regularly exceeds 82 F (28 C), halides in a closed canopy, or a humid climate where evaporation barely works. Size it from the manufacturer's chart and go one size up if the room runs hot, because an undersized chiller runs continuously and never reaches set point.
If your tank hits 81 F twice a summer, a chiller is an expensive way to solve a problem a fan would handle. If it sits at 83 for six weeks, buy the chiller.
The heater is the bigger risk
Over the life of a tank, the heater is a bigger threat than the summer, because the failure mode is worse.
Heaters fail in two ways. They fail off, and the tank cools to room temperature over a day or two, which you notice and fix. Or they fail stuck on, the thermostat contacts weld closed, and the heater runs until the tank reaches whatever temperature its wattage can drive it to. A 300 W heater on a 200 litre (50 gallon) tank can take it past 90 F (32 C) overnight, and by morning the tank is gone. This is the classic tank killer.
Two things prevent it, and you want both.
Two smaller heaters instead of one big one. Size the total at roughly 1 W per litre (3 to 5 W per gallon) and split it: two 150 W heaters instead of one 300 W. If one sticks on, it can only push the tank a few degrees above what the room would hold, slowly enough that a daily check catches it.
A controller. An external temperature controller with its own probe sits between the wall socket and the heaters and cuts power at set point. Set the heater's own thermostat a degree above the controller. If the heater sticks on, the controller cuts it. If the controller fails on, the heater's thermostat still stops it. A 30 dollar plug in controller does the job. Check its probe against a second thermometer twice a year, because probes drift.
If the tank is at 86 tonight
The order to work in:
- Lights off, lid off, fan on.
- Add air. An airstone, or a powerhead aimed at the surface. Low oxygen is what actually kills at high temperature, and night is when it runs out.
- Float sealed bottles of frozen water in the sump. Never put ice or cold water directly into the tank, because it drops salinity unpredictably and creates a cold pocket next to whatever it lands on.
- Bring it down slowly. Aim for 1 to 2 F (0.5 to 1 C) per hour, because a crash from 86 to 78 in an evening is a second stress on top of the first.
- Do not do a water change unless the new water is within a degree of the tank and matched for salinity, because a large change of cold, mismatched water is how people turn a hot tank into a dead one.
Then find out why it happened, because it will happen again.
Log the temperature, every day
Temperature is the one reading that should be daily, and the one people skip, because it does not use a test kit. A tank that reads 78 F at 7am and 82 F at 6pm has a swing problem you will never see if you only look in the morning. So look twice, same times, same probe, and write both down.
A line on a chart shows you what a probe cannot: that the summer peak has been creeping up half a degree a week since June. Log salinity alongside it, because the fan is moving that too. The reef tank testing schedule has the cadence for everything else.
Get the spreadsheet
Make a copy in Google Sheets (needs a Google account)
Download the Excel version (no account needed)Free, no email, no attribution. Readings turn green inside your target band and red outside it, and the charts draw your target band as dashed lines.
Tracking a summer by hand means two temperature readings a day plus a salinity check, in a notebook that never shows you the swing getting wider. Recif is the reef companion app we built for exactly this: it logs every test, charts each parameter against your own target ranges so a widening daily swing or a creeping salinity line is obvious, works out consumption trends, reminds you when a parameter has gone too long untested, and rolls the whole tank into one Stability Score. It is available on both the App Store and Google Play.