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Worst case: Act today5 possible causes

Fish is not eating

A fish ignores food it previously took, or has never eaten since being added.

Short answer

Refusing food is one of the earliest and most general signs that something is wrong. Water quality, stress, the wrong food and illness all produce it, and the pattern — one fish or all of them, new or established — narrows it quickly.

Check these first, in this order

  1. Test ammonia, nitrite and nitrate.
  2. Is it one fish or all of them?
  3. How long has the fish been in the tank?
  4. Is the food appropriate for the species and does it reach the fish?

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 problem

Act todayvery common

How to tell: All or most fish affected together. Testing confirms it.

Check

  • Ammonia
  • Nitrite
  • Nitrate
  • Temperature

Response

  • Water change
  • Correct the underlying cause
  • Resume feeding once parameters are clean

Illness or internal parasites

Act todaycommon

How to tell: One fish, established in the tank, refusing food while others eat — particularly with weight loss, white stringy faeces, or a sunken belly.

Check

  • Body condition compared with tankmates
  • Faeces appearance
  • Whether the fish is wild-caught

Response

  • Quarantine if possible.
  • Wild-caught fish frequently carry internal parasites; this is a case for veterinary advice rather than guesswork.

Wrong food for the species

Act within dayscommon

How to tell: The fish is one that commonly refuses dry food — pea puffer, scarlet badis, glass catfish, bumblebee goby, many wild-caught species — or is a bottom-dweller being fed floating flake.

Check

  • Species dietary requirements
  • Whether food reaches the fish's feeding zone

Response

  • Offer frozen or live foods appropriate to the species.
  • Use sinking food for bottom-dwellers and feed after lights-out for nocturnal species.

Outcompeted at feeding time

Act within dayscommon

How to tell: One slow or timid fish in a tank with faster ones. It approaches food and gets there too late.

Check

  • Watch a feeding session closely
  • Note which fish reach the food first

Response

  • Target-feed with tongs or a pipette
  • Feed in two places at once
  • Reconsider the combination

Newly added and still settling

Observe and monitorvery common

How to tell: Fish added within the last week. Otherwise normal appearance and no other symptoms.

Check

  • How long since it was added
  • Whether it hides constantly

Response

  • Dim the lights and add cover.
  • Offer a small amount of a tempting food such as frozen bloodworm or live food.
  • Most fish resume feeding within a few days. Remove uneaten food.

What not to do

  • Continuing to add food that is not being eaten, which then decays and worsens the water

When to involve a vet

  • Progressive weight loss despite eating
  • Refusal lasting more than a few days in an established fish
  • Accompanied by bloating or abnormal faeces

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

  • Quarantine new fish
  • Match food to species
  • Feed conservatively and observe every feed