Koi or goldfish lying on its side: sleeping sickness, swim bladder disorder, or an emergency?

Seeing a koi motionless, lying on its side at the bottom of the pond, is one of the most distressing sights for an owner. You may think it is dead, then see it move a fin. A fish lying on its side is not a disease in itself; it is a symptom, and its causes range from a simple digestive problem to a life-threatening emergency. Here is how to tell the difference within minutes, and what you can do to save it.

First question: one fish or several?

This is the clue that sorts through the causes fastest.

• Several fish lying down or lethargic at the same time: the cause is collective. The water, oxygen, or an infectious agent that is spreading.
• One fish: the cause is individual. Swim bladder, injury, internal infection, age.

Collective causes

— A water or oxygen problem

This is the first thing to check because it is the most urgent and the most common. Nitrite or ammonia levels above safe thresholds, a sudden pH drop, or a lack of oxygen in the early morning can leave fish completely lethargic. They breathe rapidly, stay near the bottom or, conversely, at the surface, then eventually end up on their sides.

— Koi sleeping disease

Its cause is a virus, the Carp Edema Virus (CEV), identified in Japan in the 1970s and now present in Europe. Affected koi lie on the bottom, often on their sides, as if asleep. Their eyes are sunken, their gills swollen and damaged, and they may have skin lesions. The disease mainly strikes between 15 and 25 °C, often after transport or the arrival of new fish, and can kill a large proportion of a group within a few days.

There is no medication for the virus itself. However, a salinity of about 0.5% (5 g of non-iodized salt per liter), maintained for several days, significantly improves survival: it relieves the gills and helps the fish regulate its water balance. Such a dosage should be used in a quarantine tank or in a pond without sensitive plants, ideally after confirmation by a veterinarian, because the diagnosis is made through laboratory analysis.

— Koi herpesvirus (KHV)

Even more serious, it also causes lethargy, necrotic gills, and sunken eyes, with very high mortality between 18 and 28 °C. Any group mortality of koi with these signs warrants veterinary advice and an analysis.

Individual causes

— The swim bladder

The swim bladder is the air-filled sac that allows fish to regulate their buoyancy. When it malfunctions, the fish floats head down, turns onto its back, or remains stuck on its side at the bottom. Fancy goldfish with round bodies, such as lionheads, orandas, or Chinese veiltails, are particularly prone to this because their organs are compressed into a body shortened through selective breeding. Causes include floating food swallowed with air, constipation, water that is too cold, and sometimes an infection.

What helps:

1. A two- to three-day fast.
2. Then, a few shelled and cooked peas, which aid digestion.
3. Switch to sinking food, soaked for a few seconds before feeding.
4. Slightly warmer water in a shallow isolation tank, so the fish does not have to struggle.

— An internal infection or dropsy

If the fish lying down also has a swollen belly or raised scales, the condition is more serious. Isolate it and seek advice: the prognosis is guarded.

— Shock or age

A fish that has hit a wall while fleeing a predator, or experienced a sudden temperature change, may remain stunned for a few hours. A very old fish may also fade away this way, slowly.

The isolation tank, your best tool

A simple 100 to 200-liter tank, filled with pond water and equipped with an air pump, allows you to observe the fish closely, treat it without treating the entire pond, and protect it from others. Cover it with a net: a weakened fish may also jump. Change one-third of the water every two days.

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Frequently asked questions

— My koi is lying on its side: is this sleeping disease?

This is possible if several koi are lethargic at the bottom, with sunken eyes, in water between 15 and 25 °C. But check the water and oxygen first, as these are much more common. Only a laboratory analysis can confirm the disease.

— My lionhead goldfish is swimming upside down; what should I do?

Isolate it in a shallow, slightly warmer tank, fast it for two to three days, then offer it shelled peas and sinking food. Most cases improve within a week.

— My goldfish is lying on its side at the bottom; is it going to die?

Not necessarily. If it is breathing and responsive, act quickly: test the water, aerate, and isolate it. A fish that no longer responds at all and whose gills are barely moving, however, is nearing the end of its life.

— Can salt be added to the entire pond?

At a low dose, yes, but plants do not tolerate it well and it is only removed through water changes. For therapeutic doses, a quarantine tank is always preferable.

In summary

• Several fish lying down: water, oxygen, or virus.
• A round goldfish upside down: swim bladder, most often.
• Water test, aeration, isolation, fasting: the first four steps.
• Sleeping disease: salinity of approximately 0.5% in quarantine and veterinary advice.

Last updated: September 28, 2026.