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Worst case: Act now5 possible causes

Ammonia detected in the tank

An ammonia test returns a reading above zero in a tank that should be cycled.

Short answer

Ammonia above zero in a stocked tank means production has outrun the filter. Change water now, stop feeding, and then find out why — because the reading is the symptom, not the cause.

Check these first, in this order

  1. Confirm the reading with a second test, and check the reagent expiry date.
  2. Test pH — it determines how much of the reading is the toxic free-ammonia form.
  3. Check whether the filter is running and whether it has recently been cleaned or replaced.
  4. Look for a dead fish, uneaten food or decaying plant matter.

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.

The tank is not cycled

Act nowvery common

How to tell: The tank is less than about eight weeks old, or fish were added before cycling finished.

Check

  • Tank age
  • Whether a fishless cycle was completed
  • Nitrite and nitrate readings — both zero suggests no cycle at all

Response

  • Daily water changes large enough to keep ammonia below 0.25 mg/L.
  • Feed minimally.
  • Seed the filter with mature media from an established tank if you can get it.
  • Do not add more fish until ammonia and nitrite hold at zero.

Filter capacity lost

Act nowcommon

How to tell: An established tank that was fine last week. Media was replaced or rinsed under the tap, the filter was off for a period, or medication was dosed into the display tank.

Check

  • Recent filter maintenance
  • Recent medication
  • Whether the filter stopped at any point

Response

  • Water change and reduced feeding while the population recovers.
  • Never replace all media at once; rinse in tank water only.

Something is decaying in the tank

Act todaycommon

How to tell: An established tank with no filter changes. Often a missing fish, a dead snail, a buried piece of food or a large plant that melted.

Check

  • Count the fish
  • Check behind and under hardscape and equipment
  • Smell the substrate in suspicious areas

Response

  • Remove the source
  • Water change
  • Vacuum the affected substrate area

Overstocking or overfeeding

Act within dayscommon

How to tell: Ammonia appeared gradually after new fish were added or feeding increased.

Check

  • Recent additions
  • How much food is being given and whether any is left after two minutes

Response

  • Reduce feeding to what is consumed in a minute or two, once a day.
  • Increase water change frequency while the filter catches up.
  • Reconsider the stocking plan rather than compensating indefinitely.

The reading is an artefact

Observe and monitoroccasional

How to tell: A dechlorinator that binds ammonia was used recently, or the supply uses chloramine. Fish appear entirely normal and nitrite is zero.

Check

  • Whether an ammonia-binding conditioner was used
  • Whether the supply uses chloramine
  • Whether fish show any symptoms at all

Response

  • Retest in 24 hours.
  • Understand that a bound-ammonia reading is not the same as free ammonia, but do not assume it is safe without checking the fish.

What not to do

  • Raising pH while ammonia is present — it converts harmless ammonium into toxic free ammonia.
  • Adding more bottled bacteria instead of changing water. Dilution acts now; bacteria take days.
  • Cleaning the filter thoroughly in response, which removes the capacity you need.

Prevention

  • Cycle fishless before stocking
  • Stock gradually
  • Feed conservatively
  • Rinse media in tank water only