SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/fish-safe-zero

What levels of ammonia and nitrite are safe in a cycled aquarium?

Zero parts per million, or undetectable by a test kit.

Start playing free

  1. 50 parts per million
  2. 10 parts per million
  3. Zero parts per million ✔

Why: Any measurable ammonia or nitrite can harm fish. A cycled tank should show 0 ppm for both.

Huh, didn't know that: Nitrite can harm fish at levels as low as 0.10 mg/L.

The dad joke
What did the test kit say to ammonia? Zero tolerance.
Like this card?
Play the game

References

Answer: Zero parts per million. Any measurable ammonia or nitrite can harm fish. A cycled tank should show 0 ppm for both.

  1. UF/IFAS Extension FA031: Ammonia in Aquatic Systems edis.ifas.ufl.edu/publication/FA031
    In healthy ponds and tanks, ammonia levels should always be zero.
    Checked 2026-10-11.
  2. UF/IFAS Extension FA031: Ammonia in Aquatic Systems edis.ifas.ufl.edu/publication/FA031
    Nitrite is toxic to fish at levels as low as 0.10 mg/L.
    Checked 2026-10-11.

Think this answer is wrong? Tap "Challenge this answer" on the card in the game and tell us why.

Test yourself in the game

Fish & AquariumsWhat causes new tank syndrome?Fish & AquariumsWhich water tests matter most during a new tank cycle?Fish & AquariumsHow can overfeeding worsen water quality?

That's stuff everyone should know.