Start by removing the fish, then check what could still harm the others: temperature, oxygen and surface movement, salinity, and ammonia or nitrite in a new tank. Then look at disease, aggression and jumping. Write down what you find. A single loss often has no clear cause, but working through the list protects the rest of the tank.
First: remove it and protect the others
Remove the fish straight away. Bulk Reef Supply notes that a dead fish adds a sudden load of organic matter, which can trigger a bacterial bloom. If other fish look stressed, breathing fast or hanging at the surface, increase surface movement now.
The checklist
Work down the list. Most causes show up in the first four.
- Temperature. Too hot cuts oxygen. Check on a second thermometer and look for a spike in your log. See reef tank too hot.
- Oxygen. Was a pump off, a power cut, or the water cloudy? See cloudy water and power cuts.
- Salinity. A failed top-off or a manual top-up mistake.
- Ammonia and nitrite. Mainly in tanks under a few months old, or after a big loss of bacteria. Test both.
- Disease. Spots, cloudy eyes, rapid breathing in other fish. New fish without quarantine are the usual source.
- Aggression. Torn fins, a fish hiding constantly, a recent addition that bullies.
- Jumping. Check the floor and sump. Many fish jump, especially at night or when chased.
- Something added. A new coral, a treatment, sprays or cleaning products near the tank.
Use your records
Look back two weeks: tests, water changes, new additions, dosing changes, equipment problems. A test log with dates often points straight at the cause, or rules out the water entirely.
If you can’t find a cause, test anyway and log it. Bulk Reef Supply lists acting on a single test as a common mistake, so confirm anything surprising before you change things.
When to get help
If more than one fish dies, or others show symptoms, contact an experienced local fish shop or reef club with your test results and a photo. Disease spreads fastest when it isn’t identified.
Common questions
Should I do a water change after a fish dies?
If you’re unsure of the cause, a water change with matched saltwater is a safe step. Test first, so you have a record.
Can a dead fish poison the tank?
A dead fish breaking down adds ammonia and organic waste. Remove it as soon as you find it.
Why did my new fish die?
Stress from transport, disease or aggression are common. Quarantine and slow acclimation help.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. It can’t see your tank: if livestock is in distress, act first, then ask an experienced local shop or club, ideally with your test log to hand.
- Bulk Reef Supply: what causes cloudy aquarium water
- Charterhouse Aquatics: heatwave survival guide
- Bulk Reef Supply: 10 common water testing mistakes
Keep going
Log tests to rule causes out · Free Reef Companion · All calculators