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Worst case: Act today4 possible causes

Fish is bloated

The abdomen is distended. Whether the scales stand out from the body is the critical distinction.

Short answer

Bloating has several very different causes, and the one thing to check first is whether the scales are raised. Scales standing out from the body — 'pineconing' — indicates fluid accumulation and carries a poor prognosis.

Check these first, in this order

  1. Look at the fish from above. Are the scales raised so the outline looks like a pine cone?
  2. Is the fish a female of a livebearing species?
  3. Is it eating, and what does the faeces look like?
  4. Test the water.

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.

Dropsy — fluid accumulation from organ failure

Act todayoccasional

How to tell: Scales visibly raised when viewed from above. Usually accompanied by lethargy and loss of appetite.

Check

  • View from directly above
  • General condition

Response

  • Isolate the fish.
  • Dropsy is a sign of underlying organ failure, often kidney, rather than a disease in itself. Prognosis is poor and veterinary advice is appropriate, including on humane euthanasia.
  • Review water quality, as chronic poor conditions are a common contributor.

Constipation or dietary bloating

Act within dayscommon

How to tell: Scales flat, reduced or absent faeces, protein-heavy diet — a common problem in Mbuna and goldfish.

Check

  • Diet
  • Faeces
  • Species' natural diet

Response

  • Fast for two days
  • Feed vegetable matter
  • Correct the long-term diet

Internal parasites

Act within daysoccasional

How to tell: Bloating with weight loss elsewhere, white stringy faeces, wild-caught fish.

Check

  • Faeces appearance
  • Body condition behind the head
  • Whether the fish is wild-caught

Response

  • Quarantine
  • Seek veterinary advice on appropriate treatment

Gravid female

No action neededvery common

How to tell: A female livebearer or egg-laying female in condition. Smooth, even swelling, scales flat, fish behaving and feeding normally.

Check

  • Species and sex
  • Whether the swelling is even and the fish is otherwise normal

Response

  • No action. Provide cover if fry are expected.

When this is not a problem at all

Checked before anything else, because a keeper who learns their swollen fish is carrying eggs and their spotted fish is a breeding male has been spared two unnecessary medications.

  • A gravid female

    Not a problemvery common

    How to tell it apart: Swollen, smooth-scaled, feeding normally and behaving normally. Check the sex of the fish and whether males are present before treating anything — medicating a pregnant livebearer is a common and entirely avoidable mistake.

  • A mouthbrooding female

    Not a problemcommon

    How to tell it apart: In cichlids, the distension is in the throat pouch rather than the abdomen, and refusing food is exactly what she should be doing.

  • A well-fed fish

    Not a problemvery common

    How to tell it apart: Round in the evening and slim by morning is a feeding pattern, not a condition.

Named conditions this could be

Each entry states the observation that separates it from the others, and links to the full record. Work through them before deciding what you are treating.

  • Dropsy as a presentation

    Named conditioncommon

    How to tell it apart: Scales standing out from the body, seen from directly above. Read the full record before acting: this is an end-stage sign of osmoregulatory failure with several possible causes, not a disease to be treated as one.

  • Constipation and intestinal blockage

    Named conditionvery common

    How to tell it apart: Swelling with the scales lying flat, in a fish still feeding, improving on a change of diet.

  • Malawi bloat

    Named conditionoccasional

    How to tell it apart: A rift lake cichlid, particularly a specialised algae grazer, that stops eating and swells over a day or two. Moves fast, and the diet is the first thing to examine.

  • Bacterial septicaemia

    Named conditionoccasional

    How to tell it apart: Swelling with reddening, vascular streaking or ulceration, and rapid deterioration.

What not to do

  • Treating every bloated fish for dropsy
  • Salt baths as a routine response without a diagnosis

When to involve a vet

  • Raised scales
  • Bloating with lethargy or appetite loss
  • Multiple fish affected

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

  • Species-appropriate diet
  • Stable clean water
  • Quarantine new arrivals