Freshwater fish temperature chart
| Fish | °C | °F |
|---|---|---|
| Betta (Siamese fighting fish) | 24–30 | 75–86 |
| Fancy goldfish | 15–24 | 59–75 |
| Common or comet goldfish | 4–24 | 39–75 |
| Guppy | 22–28 | 72–82 |
| Molly | 24–28 | 75–82 |
| Platy | 20–26 | 68–79 |
| Neon tetra | 20–26 | 68–79 |
| Cardinal tetra | 23–29 | 73–84 |
| Zebra danio | 18–25 | 64–77 |
| Harlequin rasbora | 22–28 | 72–82 |
| Bronze corydoras | 22–26 | 72–79 |
| Bristlenose pleco | 22–27 | 72–81 |
| Freshwater angelfish | 24–30 | 75–86 |
| Discus | 28–31 | 82–88 |
| German blue ram | 26–30 | 79–86 |
| Dwarf gourami | 22–28 | 72–82 |
| White cloud mountain minnow | 14–22 | 57–72 |
| Koi | 4–25 | 39–77 |
Saltwater aquarium temperature chart
| Fish or tank | °C | °F |
|---|---|---|
| Reef tank (corals) | 24.4–28.3 | 76–83 |
| Ocellaris or percula clownfish | 24–27 | 75–81 |
| Regal blue tang, yellow tang | 22–27 | 72–81 |
| Royal gramma | 22–27 | 72–81 |
| Firefish goby, watchman goby | 23–27 | 73–81 |
| Banggai cardinalfish | 23–27 | 73–81 |
| Mandarin dragonet | 22–27 | 72–81 |
| Humu Picasso triggerfish | 22–25.5 | 72–78 |
Shrimp, snail, frog and axolotl temperatures
| Animal | °C | °F |
|---|---|---|
| Cherry shrimp, Amano shrimp | 18–27 | 64–81 |
| Crystal red shrimp | 18–24 | 64–75 |
| Sulawesi cardinal shrimp | 27–30 | 81–86 |
| Mystery snail | 20–28 | 68–82 |
| Nerite snail | 22–27 | 72–81 |
| African dwarf frog | 22–28 | 72–82 |
| Axolotl | 16–18 (never above 24) | 61–64 (never above 75) |
General ranges when you do not know the species
| Aquarium type | Guide range | Basis |
|---|---|---|
| Tropical freshwater | 22–27 °C (72–81 °F) | OATA: tropical species generally need at least 22 °C |
| Coldwater / unheated indoor | 17–24 °C (63–75 °F) | OATA: unheated tanks should never fall below 17 °C |
| Marine (fish only) | 22–26 °C (72–78 °F) | LiveAquaria’s usual marine fish range |
| Reef | 24.4–28.3 °C (76–83 °F) | Holmes-Farley, Reefkeeping |
| Brackish | 24–28 °C (75–82 °F) | Shared range of common brackish species |
| Pond or koi pond | 4–24 °C (39–75 °F) | OATA pond care sheet |
What the result means
A reading just outside the range is not an emergency for a healthy fish, but a reading far outside it, or one that has lasted for days, needs action. The checker grades the gap and then raises the urgency if the problem has lasted several days or the animals look stressed.
| Result | Meaning | What to do |
|---|---|---|
| Within typical range | Inside the range | No change; keep checking daily |
| Near a limit | Inside, but within 0.5 °C of a limit | Nudge the heater toward the middle |
| Slightly below / above | Up to 1 °C outside | Correct over the next few hours |
| Below / above | 1–3 °C outside | Correct today; check heater and aeration |
| Well below / above | More than 3 °C outside | Act now, but still change it slowly |
| Above the safe maximum | Past a never-exceed value from a vet care sheet (e.g. axolotl 24 °C) | Cool immediately with a fan and aeration |
Temperature stability and daily swings
The Merck Veterinary Manual notes that sudden changes of even a few degrees can weaken fish immunity, and that white spot outbreaks are more common when temperature fluctuates. Its rule of thumb for changing temperature is 1 °F, or at most 1 °C, per hour.
Small daily swings are normal. Holmes-Farley points out that a small swing is not necessarily bad in a reef tank. The checker treats a 24-hour change of up to 1 °C as stable and up to 2 °C as a small swing that is usually fine while the water stays inside the range. Larger swings, or any swing in a species with a narrow range such as discus (28–31 °C), are worth tracing: heaters cycling, sunlight, lights and room heating switching off at night are the usual causes.
Fish tank too hot or too cold: first steps
- Too hot: turn the heater down or unplug it, add aeration (warm water holds less oxygen), open the lid and point a fan across the surface, shorten the lighting and keep the tank out of direct sun. If the room itself is warmer than the upper limit, the tank cannot cool below it without a fan, air-conditioning or a chiller.
- Too cold: check the heater is plugged in, switched on and cycling, compare its dial with a second thermometer, and raise the setting gradually. If the room gets much colder than the tank at night, the heater may be too small; the heater calculator gives the wattage.
- Either way: do not pour hot water or tip ice into the tank. Change the temperature slowly and recheck in a few hours.
How accurate is your thermometer?
- Digital probe: usually the most consistent; check it against a second thermometer now and then.
- Glass: read at eye level and keep it away from the heater.
- Stick-on LCD strip: measures the glass, so room air, lights and sunlight shift the reading.
- Infrared: reads only the surface it is aimed at; pointed at the glass it measures the glass, not the water.
- Controller probe: the tank holds whatever the probe reads, so calibrate it against a trusted thermometer.
Worked examples
A betta tank reads 31 °C (88 °F). The betta range is 24–30 °C, so the reading is 1 °C above the range: “Slightly above typical range”. Turn the heater down, add surface movement and recheck in a few hours.
A community tank has neon tetras (20–26 °C), bronze corydoras (22–26 °C) and a honey gourami (22–28 °C). The shared range is 22–26 °C and the suggested setting is 24 °C. Adding discus (28–31 °C) leaves no shared range at all, so they need a separate tank.
