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Worst case: Act now4 possible causes

Fish dying in a new tank

Fish die within days or weeks of a tank being set up, often one at a time.

Short answer

In a tank under about three months old, the overwhelming probability is that it was not cycled. Test ammonia and nitrite before considering anything else.

Check these first, in this order

  1. Test ammonia and nitrite immediately.
  2. How old is the tank, and was a fishless cycle completed?
  3. Was the water dechlorinated?
  4. How many fish were added at once?

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.

Uncycled tank

Act nowvery common

How to tell: Tank under about eight weeks old with detectable ammonia or nitrite. This accounts for the great majority of cases.

Check

  • Ammonia
  • Nitrite
  • Tank age

Response

  • Daily water changes to keep ammonia and nitrite below 0.25 mg/L.
  • Stop feeding almost entirely.
  • Seed with mature filter media if you can obtain it.
  • Add no further fish until both read zero.

Chlorine or chloramine not neutralised

Act nowcommon

How to tell: Deaths within hours of filling or a water change, with clean ammonia and nitrite readings.

Check

  • Whether a dechlorinator was used and at the right dose
  • Whether the supply uses chloramine

Response

  • Treat the full tank volume immediately
  • Always dechlorinate before water enters the tank

Too many fish added at once

Act todaycommon

How to tell: A full stocking list added in the first week, even in a nominally cycled tank.

Check

  • How many fish, added when

Response

  • Water changes while the filter catches up
  • Stock in small groups a week or two apart

Temperature or acclimation shock

Act todayoccasional

How to tell: Deaths within a day of adding fish, with clean water. Often a large difference between bag water and tank water.

Check

  • Temperature
  • How the fish were acclimatised
  • Difference between shop water and tank water

Response

  • Float and then drip-acclimatise new fish
  • Match temperature carefully

What not to do

  • Adding more fish to 'replace' losses before the tank is cycled.
  • Treating for disease when the cause is water quality.
  • Doing a complete tank restart, which destroys whatever bacterial population has developed.

Prevention

  • Complete a fishless cycle before buying any fish
  • Add fish gradually
  • Dechlorinate every drop