AquariumHQ
Search
Worst case: Act within days5 possible causes

The cycle seems to have stalled

Ammonia or nitrite readings stay high for weeks with no sign of progressing.

Short answer

A stalled cycle almost always has a specific chemical cause: pH too low, ammonia dosed too high, or something inhibiting the bacteria. It is rarely a matter of waiting longer.

Check these first, in this order

  1. Test pH and KH. Nitrification slows sharply below about pH 6.5 and largely stops below 6.
  2. How much ammonia was dosed? Above about 5 mg/L is inhibitory.
  3. What ammonia source was used — was it pure, or did it contain surfactants?
  4. Check the test kit expiry, and whether the nitrate reagent is being shaken hard enough.

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.

pH too low

Act within daysvery common

How to tell: pH has drifted below about 6.5 and KH is at or near zero. Nitrification produces acid and consumes buffer, so a cycle can acidify itself to a halt.

Check

  • pH
  • KH

Response

  • Raise KH with a carbonate source or a large water change
  • Nitrification usually resumes within days

Ammonia dosed too high

Act within dayscommon

How to tell: Ammonia dosed well above 4–5 mg/L, often 'to be sure'. High concentrations inhibit the organisms.

Check

  • Dosing history
  • Current ammonia reading

Response

  • Large water change to bring ammonia to 2 mg/L or below
  • Resume dosing at the lower target

Unsuitable ammonia source

Act within daysoccasional

How to tell: Household cleaner used rather than pure ammonia; surfactants foam when shaken.

Check

  • Shake a sample — persistent foam indicates surfactants
  • Read the ingredients

Response

  • Water change and switch to pure ammonium chloride

Temperature too low

Observe and monitoroccasional

How to tell: Unheated tank in a cool room. Nitrification slows markedly below about 20 °C.

Check

  • Temperature

Response

  • Raise to 26–28 °C for the duration of the cycle

The cycle is not actually stalled

No action neededcommon

How to tell: Nitrite is high but nitrate is rising too. The second stage takes longer than the first and this is normal.

Check

  • Nitrate reading, tested properly with vigorous shaking

Response

  • Continue. The nitrite stage commonly lasts one to three weeks.

What not to do

  • Repeatedly adding bottled bacteria without addressing pH
  • Restarting the tank from scratch

Prevention

  • Maintain KH above about 4 °dKH during cycling
  • Dose ammonia to 2–3 mg/L, not higher
  • Use pure ammonium chloride