Aquarium temperature: which one for which fish
Updated on August 26, 2026 · ~6 min read
A tropical freshwater tank is fine between 24 and 27 °C (75-81 °F), a reef between 24 and 26 °C (75-79 °F), discus higher (28-30 °C, 82-86 °F). Fast swings stress the animals more than a value slightly off ideal: stability matters more than the decimal.
Temperature is the easiest parameter to measure and the most underestimated. It governs the metabolism of fish and corals, decides how much oxygen stays dissolved — the warmer the water, the less there is — and sets the speed at which a problem degenerates: the same ammonia at 22 °C (72 °F) and at 29 °C (84 °F) does not do the same damage in the same time.
Reference values
| Tank / inhabitants | Temperature |
|---|---|
| Tropical freshwater community | 24-27 °C (75-81 °F) |
| Discus | 28-30 °C (82-86 °F) |
| Neocaridina shrimp | 20-24 °C (68-75 °F) |
| Caridina shrimp (crystal, taiwan bee) | 20-23 °C (68-73 °F) |
| African rift lake cichlids | 24-26 °C (75-79 °F) |
| Goldfish and coldwater tanks | 18-22 °C (64-72 °F) |
| Betta | 25-28 °C (77-82 °F) |
| Reef and marine | 24-26 °C (75-79 °F) |
| Brackish | 24-27 °C (75-81 °F) |
Two rows deserve a note. Shrimp do better cooler than most community tanks are kept: above 26 °C (79 °F) they live faster, breed less and moulting gets riskier. A goldfish is not a tropical fish: kept at 26 °C (79 °F) it burns through its years, and it does not belong in a heated tank with tetras — quite apart from reaching 25 cm (10 in) and wanting at least 100 litres (26 gal).
Stability matters more than the value
A fish rated for 24-27 °C (75-81 °F) held steadily at 28 °C (82 °F) is better off than one swinging between 24 and 27 every day. Swings force the body to re-adapt its metabolism continuously, and they are one of the three factors — with ammonia and transport — that open the door to white spot disease: not because they "create" the parasite, but because they lower the defences of a fish that already carried it.
Hence: temperature is not corrected abruptly. One degree per hour is already fast. Anyone finding the tank at 30 °C (86 °F) should not tip cold water into it: you bring it down gradually, and meanwhile you increase surface movement.
In summer: when the water heats up
It is the most common seasonal problem in warm climates, and it is not merely an annoyance: above 29-30 °C (84-86 °F) in a freshwater tank the dissolved oxygen falls while the fish's metabolism — that is, their oxygen consumption — rises. The two curves go in opposite directions, which is why the first symptom of heat is a fish at the surface with its mouth open.
What works, in order of effectiveness against cost:
- A fan across the surface. Evaporation cools: a computer fan aimed at the water surface is worth 2-3 °C (4-5 °F), which is nearly always what is needed. ⚠️ Water evaporates much faster, so you top up daily — with RO water, or you concentrate salts and hardness (in a marine tank, the salinity).
- Lid open and lights reduced. Light fixtures heat, older ones especially. Shorten the photoperiod during the hottest hours, or move it to the evening.
- More surface movement. It does not cool, but it puts oxygen back where it is needed.
- Bottles of frozen water, as an emergency measure and with the bottle sealed. They drop the temperature in steps, so they are the worst of the remedies that work: prefer the fan.
- A chiller, if the tank is a reef or holds expensive animals. It costs as much as a household appliance and it draws power, but it is the only solution that holds a fixed value.
And one thing that does not work: adding oxygen with an air pump while the water is at 31 °C (88 °F). Oxygen does not go in beyond saturation, and saturation at that temperature is already low. The problem is the temperature, not the air.
In winter: the heater
A heater is sized at around 1 W per litre in a normally heated room, more in a cold one. Undersizing it keeps it on permanently; oversizing it makes it riskier if the thermostat sticks in the "on" position — which is the classic way a tank is lost overnight.
Three precautions that cost nothing:
- Do not trust the heater's own dial. The markings are indicative: the temperature is what a thermometer in the tank says, always at the same spot.
- A second thermometer, or at least an occasional check with another instrument. A thermometer that fails does so without warning.
- Switch it off before a water change, if the level drops below its body. In air it is ruined, and glass models can crack.
How to measure it
A digital thermometer with a probe, or a strip on the glass (less precise, but always visible), always at the same spot and at the same time. In a large or poorly circulated tank the difference between bottom and surface, or between the heater's corner and the opposite one, can be a whole degree: measuring in a different place each time produces a trend that does not exist.
In summer it is worth watching the afternoon peak as well as the morning value: that is what decides whether you need to act.
The link with the other parameters
Temperature is not an isolated number, and three connections are worth having clear:
- Oxygen. Warm water holds less of it. In freshwater the reference is 6-8 ppm, in marine 6-7 ppm (salt lowers saturation); below about 4 ppm you are in an emergency.
- Ammonia. Warmer means a larger fraction of NH₃, the toxic form. In an ammonia emergency, lowering by a couple of degrees gradually reduces the damage.
- The speed of the nitrogen cycle. Nitrifying bacteria grow best between 25 and 28 °C (77-82 °F): during start-up, keeping the temperature at the top of the range shortens the wait by days.
How Gurglee does it
Gurglee keeps temperature among the basic parameters, with the band for your type of tank, and converts between °C, °F and kelvin by itself — useful if your probe or controller speaks another unit. The chart with the target band is the quickest way to tell a day/night swing from a real drift, which is the distinction you act on; and the app flags when the latest reading is out of band, with the option of asking the tank doctor starting from that tank's actual values.
Related guides
Ideal water parameters chart by type of tank
The reference values of every water parameter, for freshwater, planted, brackish, marine and reef tanks, in one chart you can come back to.
Fish gasping at the surface: what to check
A fish sitting at the surface with its mouth open has an oxygen or a gill problem: the seven causes in order of likelihood and what to do in the first few minutes.
The aquarium nitrogen cycle: what it is and how long it takes
What the nitrogen cycle is, how long it really takes and how to tell when a tank is ready for fish. With the values to check week by week.
How many fish in a 60, 100 or 200 litre tank (16, 26, 53 gal)
How many fish actually fit, with the calculation model explained and its assumptions: capacity in centimetres of fish per litre, by type of tank.