Emergencies

Reef tank emergencies: what to do first

Pick what is happening. Each page starts with short numbered steps you can follow on a phone, then explains why.

• Power cut

  First: Do not feed.

  Most tanks cope with two to three hours of still water. Oxygen runs out before heat does, so get water moving first.

• Heater stuck on (tank too hot)

  First: Unplug the heater and take it out of the water once it has cooled for a few minutes.

  Warm water holds less oxygen, so a hot tank is short of oxygen as well as hot. Act now, but cool gently.

• Overdose

  First: Unplug the doser, or stop adding the product.

  The danger is speed: a big, fast change in alkalinity or pH stresses corals more than a high reading held steady.

• Leak or flood

  First: If water has reached sockets or extension leads, do not touch them. Switch off at the consumer unit (fuse box).

  Water and mains electricity together are the real danger here. People first, then the tank.

• Top-off (ATO) failure

  First: Unplug the top-off pump.

  A stuck top-off can drop salinity fast; an empty one raises it slowly over days. Both are fixable if you correct slowly.

• Things are dying (tank crash)

  First: Remove anything dead, including snails and crabs you can reach.

  When several animals die together, the cause is usually something in the water or a piece of equipment, not a disease. Work through the checks in order.

Not an emergency?

For problems that build over days, such as algae, closed corals or a fish scratching, use the symptom finder. Going away soon? Read holidays and tank sitters.

Last checked 21 October 2026. How we check