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Worst case: Act today4 possible causes

Fish is lethargic

A fish rests on the substrate or hangs listlessly, often with clamped fins, and moves little.

Short answer

Lethargy with clamped fins is a general distress sign. Test the water first — ammonia, nitrite and temperature account for most cases — then look for a specific illness.

Check these first, in this order

  1. Test ammonia, nitrite and nitrate
  2. Check temperature against the species' range
  3. Check whether fins are clamped
  4. Check whether it affects one fish or all

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

Act nowvery common

How to tell: Several fish affected simultaneously; testing confirms it.

Check

  • Ammonia
  • Nitrite
  • Nitrate

Response

  • Water change
  • Stop feeding
  • Correct the cause

Temperature outside the species' range

Act todaycommon

How to tell: Too cold makes fish sluggish and bottom-sitting; too warm makes them listless and gasping. Check the heater with an independent thermometer.

Check

  • Independent thermometer reading
  • Whether the heater is working
  • The species' actual range

Response

  • Correct the temperature gradually, not abruptly
  • Replace a failed heater

Illness

Act todaycommon

How to tell: One fish, clean water, correct temperature. Look for additional signs — spots, ulcers, bloating, abnormal faeces.

Check

  • Full physical inspection
  • Body condition
  • Breathing rate

Response

  • Quarantine if possible
  • Seek veterinary advice rather than sequential medication

Old age or end of life

Observe and monitoroccasional

How to tell: The fish is at or beyond the species' typical lifespan and has declined gradually.

Check

  • How long the fish has been kept
  • The species' expected lifespan

Response

  • Keep conditions optimal and reduce stressors
  • Consider humane euthanasia if the fish is suffering — ask a vet about the method

What not to do

  • Medicating without a diagnosis
  • Raising temperature reflexively — it lowers oxygen

When to involve a vet

  • Lethargy persists with clean water and correct temperature
  • Accompanied by ulceration, bloating or abnormal swimming

AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.

Prevention

  • Stable water
  • Correct temperature for the species
  • Quarantine new arrivals