Why does my common goldfish have a swollen abdomen?

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One of our readers has noticed his common goldfish has a very swollen abdomen. Dr Peter Burgess explains why this may have happened and what can be done to help...

Q) I’ve had my Common goldfish for many years but it has swollen up enormously in the past few months. It’s in an outdoor pond along with nine others. There are plants and a pump in the pond. The affected goldfish swims around the pond as normal, eats well, and I have seen that the food is passing through its system okay; I am just so worried it’s going to explode. Is there anything we can do for this fish?

KATHRYN EVERSON, VIA EMAIL

A) Peter says: Clearly, your goldfish is suffering from a grossly distended abdomen. Judging from the photos you sent, the swelling appears fairly symmetrical, and there are a few possible causes:

Egg-binding, known technically as dystocia, generally arises when a female fish fails to spawn. This may be due to absence of mature males or, in the case of temperate fish such as goldfish or pondfish, due to lack of environmental spawning triggers, notably too cold a temperature at spawning time. The unshed eggs, if not reabsorbed, may accumulate within the body and eventually start to break down, risking infection and possible death.

Fluid accumulation in the abdominal cavity can arise from a fluid imbalance, which in turn can be caused by damage or disease of one or more organs involved in the fish’s osmoregulatory system (its internal salt-water balance), such as a diseased kidney or gills. Organ failure, due to old age, is another, albeit natural, cause. Certain internal tumours can cause fluid accumulation, as can a condition known as polycystic kidney disease which is quite common in goldfish. A similar condition, also widely reported in goldfish, is kidney enlargement disease, which is linked to infection with an internal parasite, Hoferellus carassii, which unfortunately, is virtually impossible to treat successfully.

In the case of your fish, my best guess would be fluid accumulation due either to organ failure or an internal tumour or kidney problem. Unfortunately, none of these are easily curable.

If you have a spare water container of adequate size, you could try maintaining the fish in a weak solution of salt (sodium chloride; or better still, use ‘tonic’ salt as sold for freshwater fish). Make up the salt to 3g per litre and keep the fish in this for a week or two, to see if the swelling reduces over this time. If so, it points to fluid accumulation as the underlying cause.

A vet with a special interest in fish (try Googling “fish veterinary society” for a list), might decide to take a sample of the fluid for analysis, to check for the presence of blood or bacteria, for example. But this may not lead to a precise diagnosis or a follow up treatment strategy. 

Other than that, there sadly isn’t much you can do. If the fish is still eating well and positively engaging with its pondmates, then these are encouraging signs that it isn’t in significant pain or stress, but that could change over time, so continued close monitoring of the condition is important.