Temperature
The parameter that drives metabolic rate for everything in the tank. Stable matters more than precise.
What it is
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 it matters
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.
How to test
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.
What high and low look like
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.
How to fix
Too low — match heater wattage to tank size (a useful rule: 1 W per litre for a tropical tank in a 20 °C room). Too high — increase surface agitation, run a fan across the surface for evaporative cooling, drop the room temperature, or float a sealed bag of ice for short-term emergencies. Match the temperature of any added water within 1 °C to avoid swings during water changes.
