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Worst case: Act now3 possible causes

Nitrite detected in the tank

A nitrite test returns a reading above zero.

Short answer

Nitrite means the first stage of the nitrogen cycle is working and the second is not keeping up. Change water, stop feeding, and add chloride only if the tank has no invertebrates or salt-sensitive fish.

Check these first, in this order

  1. Test ammonia too — the pattern tells you where the cycle is
  2. Check filter operation
  3. Check recent stocking and feeding changes

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.

Tank still cycling

Act nowvery common

How to tell: Ammonia has fallen to zero or near zero while nitrite is rising. This is the normal middle phase of cycling.

Check

  • Ammonia reading
  • Tank age
  • Nitrate reading — a rise means the second stage is starting

Response

  • Water changes to keep nitrite below about 0.25 mg/L.
  • Minimal feeding.
  • Patience. This phase typically lasts one to three weeks.

Bioload increased faster than the filter

Act todaycommon

How to tell: Established tank; several fish added at once, or feeding stepped up.

Check

  • Recent additions
  • Feeding volume

Response

  • Water change
  • Reduce feeding
  • Add stock more slowly in future

Filter disturbed or medicated

Act todaycommon

How to tell: Follows filter maintenance, a power cut, or an antibacterial treatment in the display tank.

Check

  • Recent maintenance or medication
  • Whether flow has dropped

Response

  • Water change
  • Reduced feeding
  • Avoid further disturbance until readings clear

What not to do

  • Adding salt without checking the tank. Chloride reduces nitrite uptake by fish, but harms many plants, most invertebrates and several scaleless fish.
  • Assuming nitrite is less serious than ammonia. It is not.

Prevention

  • Complete a fishless cycle
  • Add stock in small steps with a week or two between
  • Keep a spare mature sponge in the filter