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Worst case: Act within days5 possible causes

Fish is hiding constantly

A fish stays out of sight almost all the time, including at feeding.

Short answer

Constant hiding is usually a social or environmental problem rather than a disease. The commonest single cause is a shoaling species kept in too small a group.

Check these first, in this order

  1. How many of its own species are present, and what is the minimum group for that species?
  2. Is there a larger or more aggressive fish present?
  3. Is there enough cover — planting, floating plants, shade?
  4. Test water quality.

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.

Water quality or illness

Act todayoccasional

How to tell: The fish previously behaved normally and has changed. Other symptoms may be present.

Check

  • Ammonia, nitrite, nitrate
  • Physical appearance
  • Whether other fish are affected

Response

  • Correct water quality
  • Observe closely for other signs

Group too small for a social species

Act within daysvery common

How to tell: The species is a shoaler or schooler and there are fewer than its minimum group. Corydoras in twos, tetras in threes, kuhli loaches singly.

Check

  • Species minimum group size
  • How many are present

Response

  • Increase the group to at least the minimum
  • Expect behaviour to change within days

Insufficient cover

Act within daysvery common

How to tell: Sparse tank, bright light, no floating plants. Affects timid species most.

Check

  • Amount of planting and hardscape
  • Light intensity and whether the surface is covered

Response

  • Add planting, wood and floating plants.
  • Counter-intuitively, more cover usually means the fish is seen MORE, not less — it feels safe enough to come out.

Bullying by a tankmate

Act within dayscommon

How to tell: One fish is being chased or blocked from a part of the tank. Often intermittent and easy to miss.

Check

  • Watch quietly for ten minutes, several times
  • Look for fin damage or missing scales

Response

  • Rehome the aggressor or the target
  • Break up sightlines with hardscape
  • Reassess the combination

Normal behaviour for the species

No action neededcommon

How to tell: Kuhli loaches, many catfish and most loaches are nocturnal or crepuscular. A newly added nocturnal fish may not be seen for weeks.

Check

  • Species activity pattern
  • Whether it appears after lights-out

Response

  • Observe after dark with a dim red light
  • Feed after lights-out

What not to do

  • Removing cover so you can see the fish, which makes the hiding worse

Prevention

  • Buy social species in proper groups
  • Plant and shade the tank
  • Check compatibility before adding