Popeye fish: causes of bulging and cloudy eyes, and what to do

Popeye (exophthalmia) is when one or both of a fish's eyes bulge out because fluid builds up behind the eye. If only one eye is affected, it's usually an injury; if both eyes bulge, think water quality or infection. Either way, test and improve the water first, because that alone fixes many cases.

Cloudy eye, where the eye turns milky white, often goes with popeye or comes before it. Both can heal over weeks to months, but the eye can be permanently damaged if the cause isn't dealt with.

How to identify it

Popeye: one or both eyes stick out further than normal, sometimes dramatically. Look at the fish from above; both eyes should stick out equally.
Cloudy eye: a white, grey or milky film over the eye. The fish may bump into things or miss food.
Blood in the eye or a red ring around it.
Other signs to look for: bloating or raised scales (dropsy), red streaks, ulcers, weight loss, not eating, or hiding.

Normal bulging eyes: some fish have big, protruding eyes on purpose: telescope-eye and black moor goldfish, bubble-eye goldfish and some other fancy breeds. Compare with a photo of the breed. Normal cloudy eyes: very old fish can develop cataracts slowly.

When it's a problem

Early: one eye slightly swollen after a bump, with the fish eating normally.
Serious: both eyes swollen, cloudy eyes that don't clear within a few days, or several fish affected.
Emergency: the eye looks like it might burst, there's bleeding, or the fish also has bloating, raised scales, red sores, is gasping, or has stopped eating. Act today.

What you see	Most likely causes
One eye	Injury: a net, a knock on decor, a fight, or a fish startled into the glass
Both eyes	Long-term poor water, bacterial infection spread through the body, or gas bubble disease
Cloudy eyes	Poor water, chlorine exposure, injury, rough handling or infection

What to do right now

Test the water: ammonia, nitrite, nitrate, pH and temperature.
Do a water change of 30 to 50% with dechlorinated water at the same temperature. Make sure your dechlorinator also treats chloramine.
Check for injuries and causes: sharp decor, aggressive tankmates (see aggressive fish), or a fish that has been jumping.
Look for tiny bubbles on the glass, fins or in the eye. These can mean the water is supersaturated with gas (gas bubble disease), for example from a leaking pump intake or very cold tap water warming up in the tank.
Move the fish to a hospital tank if it's being bullied or if you plan to treat it.
Keep the lights low and the fish calm.

What to do next

Week 1 to 2: clean water and good food. Many injury-related cases improve over one to two weeks in clean water. Keep nitrate low with regular water changes. Feed a varied, good-quality diet.

Epsom salt: uses and limits. Epsom salt is magnesium sulfate, not the same as aquarium salt. It's widely used to help draw fluid out and reduce swelling. One guide suggests 1 to 3 teaspoons per 19 litres (5 US gallons), in a hospital tank rather than the main tank. Other hobby sources use different amounts. Dissolve it in tank water before adding, and watch the fish closely.

It can reduce swelling, but it doesn't kill bacteria or fix the cause.
It makes water harder, so use it in a hospital tank, not a planted or soft-water tank.
Do partial water changes to bring it back down once the swelling goes.

If both eyes are affected or the fish is getting worse, suspect a bacterial infection inside the body. These infections need the right antibiotic for the right bacteria, ideally chosen after a vet tests a sample. Many countries now require a vet prescription for fish antibiotics, and medicated food is often better than adding medicine to the water for infections inside the body. Some infections, such as fish tuberculosis (mycobacteriosis), can't be cured, and other fish in the tank should be watched closely.

When medicating: use a hospital tank, remove activated carbon from the filter, follow the label or vet's instructions exactly, and finish the course. Take extra care with scaleless fish, plecos and other catfish, loaches, shrimp, snails and live plants, which are often more sensitive to medicine.

How long it takes: swelling can take weeks or even months to go down. Mild cloudiness often clears in days once the water improves. A badly damaged eye may stay cloudy, shrink, or be lost, but most fish manage well with one eye.

Products that help

Liquid water test kit: the first thing to reach for, as poor water causes many eye problems.
Dechlorinator that treats chloramine: chlorine exposure can cloud eyes.
Epsom salt (plain, unscented, no additives): supportive treatment for swelling in a hospital tank.
Hospital tank with heater and sponge filter: lets you treat one fish without dosing the whole tank.
Soft, fine-mesh net: reduces handling injuries.
Smooth decor and plants: fewer sharp edges to bump into.
Keep on hand: spare buckets and a thermometer for water changes.

How to prevent it

Keep up regular water changes and don't overstock. Nitrate building up is a common warning sign.
Always use dechlorinator.
Net fish gently, or guide them into a container.
Avoid sharp decor, and don't mix jumpy fish with aggressive ones.
Turn on room lights before the tank light, so fish aren't startled into the glass.
Quarantine new fish.

Why it happened

Fluid builds up behind the eye after an injury, or when bacteria or long-term poor water damage the tissue. Less often, gas bubbles form in the eye when the water holds too much dissolved gas. Popeye is a sign of a problem, not a disease in itself, so finding the cause matters.

When to get help

See a vet who treats fish if both eyes are swollen, if the eye looks close to bursting, if the fish has other signs such as bloating or sores, if several fish are affected, or if there's no improvement after two weeks of clean water. A vet can find the cause and prescribe the right medicine.

In the game

Multiple Tank Syndrome doesn't have a popeye card yet. The "Antibiotic food" product, which saves fish from "Fin rot" and "Columnaris", is the closest match to how bacterial eye infections are often treated.

Sources

Chewy (editorial). Popeye disease in fish.
Merck Veterinary Manual. Bacterial diseases of fish.
Merck Veterinary Manual. Common environmental hazards for fish (gas bubble disease, chlorine and cloudy eyes).
WikiVet. Buoyancy disorders (exophthalmos with underlying systemic disease).