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Worst case: Act now4 possible causes

Heater problem — tank too hot or too cold

Water temperature is well above or below the setting.

Short answer

Aquarium heaters fail in both directions and their dials are frequently inaccurate. A separate thermometer is the only reliable check, and a stuck-on heater can kill a tank within hours.

Check these first, in this order

  1. Read the temperature with a thermometer you trust, not the heater dial.
  2. Is the heater indicator light behaving as expected?
  3. Is the heater fully submerged and positioned in flow?
  4. Is the room temperature unusual?

Possible causes

Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.

Thermostat failed on

Act nowcommon

How to tell: Temperature well above the setting and rising; the heater light stays on continuously.

Check

  • Independent thermometer
  • Whether the light ever goes off

Response

  • Unplug the heater immediately.
  • Cool gradually — float sealed bottles of cool water, and increase aeration because warm water holds less oxygen.
  • Replace the heater. A failed thermostat does not recover.

Thermostat failed off, or heater undersized

Act todaycommon

How to tell: Temperature below the setting. Either the heater never comes on, or it runs constantly and still cannot keep up in a cold room.

Check

  • Whether the indicator ever lights
  • Room temperature
  • Heater wattage against tank volume

Response

  • Replace or add capacity
  • Two smaller heaters are safer than one large one — a single failure is then less severe

    Ambient temperature, not the heater

    Act todaycommon

    How to tell: Hot weather, direct sun on the tank, or lighting close to the surface. The heater is off and the tank is still too warm.

    Check

    • Room temperature
    • Sunlight
    • Whether the lights are hot

    Response

    • Increase surface agitation and evaporation
    • Float sealed bottles of cool water
    • Shade the tank and raise the light

    Poor placement

    Act within dayscommon

    How to tell: Temperature varies across the tank. Heater is in a dead spot or partly buried in substrate.

    Check

    • Measure at both ends of the tank
    • Check the heater is in moving water

    Response

    • Reposition near the filter outflow or intake
    • Ensure it is fully submerged to its marked line

    What not to do

    • Trusting the dial
    • Cooling a tank rapidly with cold water, which is a shock in itself

    Prevention

    • Independent thermometer
    • Two smaller heaters rather than one large one
    • Replace heaters every few years