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SESK: Stuff Everyone Should Know · sesk.info/fish-new-tank-syndrome

What causes new tank syndrome?

Ammonia and nitrite spikes in a tank without established bacteria.

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  1. Too much gravel
  2. Cold water
  3. Ammonia and nitrite spikes ✔

Why: A new filter has not grown enough nitrifying bacteria. Waste quickly builds ammonia and nitrite, which can kill fish.

Huh, didn't know that: It usually shows up within the first 6 weeks after a new tank is set up.

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References

Answer: Ammonia and nitrite spikes. A new filter has not grown enough nitrifying bacteria. Waste quickly builds ammonia and nitrite, which can kill fish.

  1. Merck Veterinary Manual: Management of Aquarium Fish www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/management-of-aquarium-fish
    Often total ammonia nitrogen (TAN) or nitrite, or both, are high enough to result in toxicity.
    Checked 2026-10-11.
  2. Merck Veterinary Manual: Environmental Diseases of Aquatic Animals in Aquatic Systems www.merckvetmanual.com/exotic-and-laboratory-animals/aquatic-systems/environmental-diseases-of-aquatic-animals-in-aquatic-systems
    because the microbial population of the biofilter has not had time to develop.
    Checked 2026-10-11.
  3. Merck Veterinary Manual: Management of Aquarium Fish www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/management-of-aquarium-fish
    usually occurs within the first 6 weeks after a new fish hobbyist sets up a tank.
    Checked 2026-10-11.

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