Goldfish Care Tips

A Free Monthly Resource For Goldfish Enthusiasts
July 2026
Issue #151

In This Issue
Polycystic Kidney Disease

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When a Goldfish develops excessive swelling, there are two likely causes: a Gonadal sarcoma or Polycystic kidney disease (PKD).

Polycystic Kidney Disease

Pond fish displaying classic PKD symptoms

I did this because after performing a necropsy on one of my Water Bubble Eyes, I found a massive Gonadal sarcoma, and these seem more common in fancy Goldfish varieties.

Subsequent necropsies done on siblings that developed swollen bellies showed polycystic kidney disease, not a Gonadal sarcoma.

There is some thought that polycystic kidney disease is more common in single-tailed varieties, and looking at posts on social media, the vast majority showing symptoms are single-tailed varieties.

This disease usually affects older fish. Mine have been between 5 and 8 years old.

Cause of Polycystic Kidney Disease

There are two schools of thought as to the cause of polycystic kidney disease:

1. Genetics, and this would be my first assumption as well, considering none of my Black Moor or Celestials have ever succumbed to the disease.
2. Parasitic attack of the kidney.

The parasite cited as the cause is a microscopic myxozoan parasite, Hoferellus carassii, that infects freshwater fish such as Goldfish and other carp species, causing polycystic kidneys and severe belly swelling.

This sounds like a possible cause, but there are a couple of problems with this theory:

1. The lifecycle of this parasite requires two hosts: fish and a worm. Spores leave the fish via urine in the spring to infect worms, which later release stages that infect fish.
2. None of my other fish varieties have ever developed this condition.

I don’t harbor the worm types that are suitable hosts in my small ponds, tubifex worms being a common one. The ponds are regularly scrubbed clean, but I do get earthworms populating my sponge filters. I keep my varieties separated, which could be the reason why no other populations of my fish have caught the disease.

Treatment Options

Unfortunately, there are none. With Gonadal sarcoma, surgery is possible if caught early enough. A Goldfish can survive without a gonad, but not a kidney.

As this swelling is gradual, it usually isn’t noticed until after the breeding season, and then egg binding is assumed the cause, or dropsy if the scales pine-cone.

The only action the owner can take is to get an X-ray of the fish to confirm what is causing the swelling, and if it is a Gonadal sarcoma, explore the possibility of surgery.

Disease Progression

As the swelling increases, internal organs are displaced. The air bladders are an example.

The fish may develop dropsy in the last stages of the disease, and have buoyancy or balance problems.

At this stage, I euthanize, as the quality of life is poor.

Palliative Care

If the diagnosis is polycystic kidney disease, appropriate care is required, which may span many months.

The fish should be kept in optimal water conditions, which includes the correct GH (general hardness) and KH (carbonate hardness), and fed a balanced high-quality diet. The disease doesn’t seem to affect appetite, but because mobility is affected, make sure the fish is getting its share of the food.

I leave the fish in with its tank mates for as long as possible, as this causes the least stress.

If you provide this environment for your fish, you know you have done all that can be done, and who knows, miracles do happen.

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