Emergency playbook

Fish or corals dying suddenly: the first hour

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.

Why each step

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

Dead animals rot quickly and release ammonia, which makes things worse.

2. Check temperature and salinity first.

They are the quickest checks and catch the commonest failures: a stuck heater, a failed top-off.

3. Test ammonia and alkalinity, then pH.

Ammonia shows rot or a filter failure; alkalinity and pH show a doser or kalkwasser problem.

4. Stop all dosing until you know the cause.

A stuck doser is a common cause, and dosing into a sick tank adds risk.

5. Do a large water change with premixed water matched for temperature and salinity.

It dilutes whatever is wrong. BRS advises keeping a premixed batch ready for exactly this.

6. Add fresh activated carbon.

Our reading: carbon removes many unknown chemicals, including some toxins.

Once the tank is stable, what next?

1. Go through every piece of equipment: heater, top-off, doser, pumps, skimmer (an overflowing skimmer can be a sign of a contaminant).
2. Think about anything new in the last 48 hours: a new animal, a new product, cleaning sprays, air fresheners, paint, someone putting their hands in the tank.
3. If zoanthids or Palythoa were scraped, cut or boiled, see the palytoxin warning below for your own safety.
4. Test again in a few hours and the next day.
5. Keep notes with times: they help you, and anyone you ask for help, find the cause.

How do I stop it happening again?

• A temperature alarm and a backup sensor on the top-off.
• Small doser bottles and a monthly doser calibration.
• Premixed salt water kept ready.
• Quarantine new fish and dip new corals.

Last checked 21 October 2026. How we check