Dropsy in koi carp: raised scales, swollen belly, what to do (and can it be saved)?

Seen from above, the fish looks like a pinecone: the scales stand up, the belly swells, and sometimes the eyes protrude from their sockets. This is the typical image of dropsy, and it is rightly alarming. Dropsy is not a disease but a symptom: a sign that the fish's body can no longer eliminate the water it constantly absorbs. The earlier action is taken, the better the chances. Here's what you need to know and do.

What happens inside the fish's body

A freshwater fish lives in an environment that is less salty than its own blood. Water therefore constantly enters its body through the gills and skin, and its kidneys work tirelessly to eliminate it. When these kidneys or other organs are affected, most often by an infection, water accumulates in the abdominal cavity and beneath the scales. The belly swells, the scales stand up, and the pressure can push the eyes outward.

This mechanism explains the usefulness of salt: by bringing the water's salinity closer to that of the fish's blood, the amount of water entering is reduced, relieving failing kidneys.

Possible causes

• An internal bacterial infection, most often caused by bacteria of the genus Aeromonas or Pseudomonas, present in all ponds and taking advantage of a weakened fish.
• Kidney damage of parasitic, tumoral, or age-related origin.
• Spring viremia in carp and koi, causing a swollen belly, hemorrhaging, and grouped deaths when the water warms in spring. It is a reportable disease: grouped deaths with these signs warrant a veterinarian's opinion.
• Persistently degraded water quality, which weakens defenses and paves the way for infections.

Dropsy or not? False alarms

Not every round belly is dropsy.

The simplest test: look at the fish from above, in grazing light. Raised scales are clearly visible from this angle, whereas they go unnoticed from the side.

Treatments that give it a chance

1. Isolate the fish in a 100- to 200-liter tank filled with pond water, aerated, filtered if possible, and covered with a net.
2. Add salt gradually until reaching 3 to 5 g/l of non-iodized salt, in two or three doses over 24 hours.
3. Treat against bacteria with a suitable antibacterial product, strictly following the instructions. For a valuable fish, a veterinarian can prescribe an antibiotic injection, which is often much more effective than a bath.
4. Maintain impeccable water quality: change one third of the water every two days, using water at the same temperature and with the same salt concentration.
5. Feed very little, with easily digestible food, only if it eats.
6. Test the main pond and monitor the other fish: if several are affected, the cause is collective.

Let's be honest: when the fish is completely pineconed and lying down, the chances are slim. If the fish is visibly suffering and does not respond after a week of treatment, ask a veterinarian how to end its suffering humanely.

Prevention is better than cure

• Stable water, tested regularly, with filtration suited to the population.
• Quarantine for any new fish.
• A diet suited to the temperature, without excess.
• Particular vigilance in spring, a high-risk period for all bacterial infections.

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Frequently asked questions

— Is dropsy contagious?

The symptom itself is not, but the bacterium or virus causing it may be. If only one fish is affected, it is most often an individual weakness. If several are affected, the cause is common.

— Is Epsom salt useful?

Some breeders add it for its effect on edema. It can complement care in an isolation tank, but it does not treat the infection causing the problem.

— How long does the treatment last?

Allow two to three weeks. The scales gradually flatten again if the treatment works. Return the fish to the pond only once it has fully recovered, gradually reducing the salinity.

— Can a goldfish have dropsy?

Yes, and it is even common in fancy goldfish. The care is the same as for a koi.

In summary

• Dropsy is a symptom, most often of an internal bacterial infection.
• Isolation, non-iodized salt at 3-5 g/l, antibacterial treatment, pristine water.
• At an advanced stage, the prognosis is very poor.
• Several fish affected: look for a common cause and seek veterinary advice.

Last updated: September 28, 2026.