Temperature

What the number controls

Water temperature, measured in degrees Celsius. Drives metabolic rate — fish, shrimp, plants, bacteria all run faster at higher temperatures and slower at lower ones. Also drives gas solubility: warm water holds less oxygen, less CO₂, and less of every dissolved gas.

Why stability beats precision

Tropical species evolved at roughly 22–28 °C. Too cold and immune systems weaken, ich (white spot) outbreaks become common, slow-moving fish look listless. Too warm and dissolved oxygen drops fast enough to stress sensitive species. Sudden swings — particularly more than 2 °C in an hour during a water change — are far more dangerous than a steady-state value sitting a degree off-target.

For planted CO₂-injected tanks, every additional degree pushes plant metabolism up and the CO₂ requirement with it. A tank that runs comfortably at 24 °C and 20 mg/L CO₂ will need closer to 28 mg/L at 27 °C to feed the same plant mass.

Temperature also sits underneath the parameters you do test. The nitrifying bacteria that run your cycle slow markedly below about 20 °C, which is why a tank cycled in a cold room stalls — the nitrogen cycle is a biological process and it runs on the same metabolic clock as everything else. And ammonia gets more toxic as temperature (and pH) rise, so the same reading that's tolerable at 22 °C bites harder at 28 °C.

The 22–26 °C band on this page is the community default, not a universal truth. Species stray in both directions, and the usual mistake is mixed livestock held at a compromise nobody likes:

Livestock	Comfortable band
Goldfish, White Cloud minnows	16–22 °C
Neocaridina shrimp	18–24 °C
Standard tropical community	22–26 °C
Discus, German blue rams	28–30 °C

A discus at 24 °C and a goldfish at 26 °C are both slowly failing, in opposite directions — pick animals that agree on a band before arguing over single degrees.

As a quick reference for the community default: below 20 °C — slow fish, plants stunted, ich risk rises. 22–26 °C — standard community tropical range. Above 28 °C sustained — fish breathing harder, dissolved oxygen dropping, planted tanks struggling. Above 32 °C — dangerous for most species; emergency action required.

Reading it honestly

A digital thermometer (cheap, accurate to 0.1 °C) or a stick-on liquid crystal strip on the outside of the glass (good enough for daily eyeballing). Pair with the heater rather than rely on it — heater dial markings are notoriously off by 2–3 °C.

Where and when you read it matters. Take the reading away from the heater and away from the filter outflow, mid-tank, where the fish actually live — a probe sitting next to the heating element reads the setpoint, not the room the tank is. Check in the late afternoon too, not just the morning: a tank in a sunlit room or under hot lights can climb 2–3 °C over the day, and it's the afternoon peak that stresses fish, not the overnight low. If you log the reading, log the time with it so you're comparing like with like.

Two cheap digital thermometers that agree beat one expensive one you've never questioned. And log temperature with every water test — the slow summer creep shows up in the trend weeks before it becomes an emergency, which is exactly the kind of drift a single reading never reveals.

When the tank runs cold

Below 20 °C fish slow down and sit low, colours dull, and appetite drops because digestion runs on temperature. This is prime ich territory — the parasite's life cycle speeds up in a stressed, chilled tank, and a cold snap after a water change is the classic trigger for an outbreak. Plants stall and stop pushing new growth, and shrimp go quiet and breed less. None of this is instantly lethal in the 15–20 °C range, but it's a slow erosion of health, and the schema only calls it critical below 10 °C.

When the tank runs hot

Above 28 °C sustained the problem is oxygen, not heat directly. Warm water simply holds less dissolved O₂, and at the same time every animal's oxygen demand is rising with its metabolism — so supply falls as demand climbs. You'll see fish breathing hard and hanging near the surface where the water is best oxygenated, shrimp especially struggling since they have little reserve. Plants may pearl heavily in the heat and then the tank crashes for oxygen overnight. Above 32 °C is dangerous for most community species and calls for emergency action.

The numbers are unforgiving: freshwater saturates at roughly 9 mg/L O₂ at 20 °C but only about 7.6 mg/L at 30 °C — a sixth less supply arriving exactly when demand peaks.

Getting heat in

Too low — match heater wattage to tank size (a useful rule: 1 W per litre for a tropical tank in a 20 °C room), and step the setpoint up gradually rather than in one jump. A heater that short-cycles or can't hold the number in a cold room is undersized; two smaller heaters at opposite ends beat one big one for even distribution and give you a fail-safe if one sticks.

The failure mode to plan for isn't a heater that quits — it's one that sticks on. A separate thermostat controller that cuts power at a setpoint costs less than the livestock it protects, and an independent thermometer is the only way to notice either failure early. Unplug heaters during water changes so an exposed element doesn't overheat and crack as the level drops.

Getting heat out

Too high — increase surface agitation, run a fan across the surface for evaporative cooling (this alone buys 2–3 °C and is the cheapest fix in a summer heatwave), drop the room temperature, or float a sealed bag of ice for short-term emergencies. Lift the lid and cut the light intensity or photoperiod while it's hot. Match the temperature of any added water within 1 °C to avoid swings during water changes — the shrimp-safe parameters guide is worth a read here, because shrimp tolerate a narrower window and react badly to exactly the kind of quick swing a careless top-up creates.

The arithmetic on water-change swings is linear: a 50% change with water 4 °C cooler drops the whole tank 2 °C on the spot — exactly the sudden swing the rest of this page warns about. Temper the new water first; the water-change impact tool runs the same mixing sums for the dissolved parameters.