Cloudy eye in fish: the water problem behind the film

What you are looking at

The eye has gone milky, grey or white — a haze over the surface, as if someone breathed on a lens — and the fish may be bumping into things, missing food, or feeding by smell along the bottom. The eye is not bulging; if it is, read the popeye guide instead, though the two go together often enough that both apply. It may be one eye or both, and that is the first thing to note, because it points at the cause.

The surface of a fish’s eye is skin, and it is covered by the same slime coat as the rest of the body. Cloudiness is that coat thickening or the cornea itself swelling in response to an irritant, and the irritant is nearly always one of these.

Both eyes, in a tank with a history. Water. Ammonia and nitrite burn the eye’s surface the same way they burn the gills; a pH that has drifted well below 6.5 or above 8 irritates it; nitrate that has sat in the 40s for weeks wears down the slime coat that protects it. This is the common case, and cloudy eyes in two or three fish at once is nearly always this.

One eye, on a fish that is otherwise fine. A scratch: from the decor, a rough net, a fight, or the glass during a chase. It clouds as it heals and clears as it does, over a week or two, as long as the water lets it.

Either, with a film that is getting thicker or fluffier. A bacterial or fungal infection on top of one of the above — a scratch that got infected, or a burn that did. The tell is that it is getting worse rather than better after a week of clean water, or that the fish is also clamped, lethargic or off its food.

A cloudy lens rather than a cloudy surface. Look closely: if the haze is inside the eye, behind a clear surface, it is a cataract. Old fish get them, as do fish fed a poor diet, and in ponds and tanks with wild-caught fish or pond snails, eye flukes — a parasite whose larvae lodge in the lens. A cataract does not clear and does not need treating; the fish adapts.

The fix

1. Test the water, today. Ammonia, nitrite, nitrate, pH and temperature. This is the whole diagnosis in most tanks, and it is the treatment too: any ammonia or nitrite above zero, or nitrate over 40 ppm, means a 30–50 % water change now and daily testing until it clears. A pH that has crashed below 6.5 usually means the KH has run out, and a series of small water changes brings both back; do not chase pH with chemicals.

2. Aquarium salt. One tablespoon per 20 litres (5 gallons), dissolved first, in the main tank if everything in it tolerates salt — most fish do; scaleless species, live plants and invertebrates do not — or in a hospital tank if not. It supports the slime coat and eases the irritation, and it is the one addition that speeds a mild case. Keep it up for a week to ten days by re-dosing for the volume of each water change, then let it dilute out.

3. Nothing else, for a week. A cloudy eye in clean, salted water clears on its own in one to two weeks, and most medications for “cloudy eye” sold in shops are a general antibacterial that the fish does not need yet. Watch the eye every day at feeding time: clearing from the edges inward is healing.

4. An antibacterial, if it is not. A film that is thicker after a week of clean water, that has turned fluffy or opaque white, or that comes with a fish that is clamped and not eating, is infected. Move the fish to a hospital tank and treat with a broad-spectrum antibacterial dosed for its volume, carbon out, the full course on the label. A fluffy white growth on the eye rather than a haze is a true fungus and takes a malachite-green or methylene-blue treatment instead.

In the app — log the test and the verdict says which readings are behind the film; a checkup with a photo of the eye and the readings tells you whether this is the water, a scratch or an infection, and Add plan to my schedule books the water changes and the salt as daily tasks.

Run a checkup on the eye →

Keeping it clear

Cloudy eye that comes back is water that went back. The reading to watch is nitrate: it is the slow one that nobody treats as an emergency, and a tank that runs it in the 30s and 40s produces a cloudy eye, a bout of fin rot or an ich outbreak every few months, one at a time, with no obvious cause. Weekly water changes sized to keep nitrate under 20 ppm are the whole prevention, and a test every week is how you know they are big enough.

Common questions

Will a cloudy eye heal on its own?

Usually, if the water is clean. A mild film clears in one to two weeks once ammonia and nitrite read zero and nitrate is under 40 ppm, and aquarium salt at one tablespoon per 20 litres speeds it. It will not heal in the water that caused it, and a film that is getting thicker after a week of clean water is infected and needs an antibacterial.

Is cloudy eye contagious?

No. It is a response to an irritant — nearly always the water, sometimes a scratch — not something one fish passes to another. Two or three fish going cloudy at once is the strongest sign that the water is the cause, because they are all in it.

Can a fish see through a cloudy eye?

Poorly, while it lasts; the fish feeds by smell and may miss food or bump into things. Sight comes back as the film clears. A cataract inside the lens is different: it does not clear, but a fish with one manages well, and even a blind fish feeds and lives normally in a tank it knows.

Why does my fish have one cloudy eye?

One eye on a fish that is otherwise well is almost always a scratch — from the decor, a net, a fight or the glass — clouding as it heals. Clean water and a little salt let it clear in a week or two. If it is getting worse instead, or the fish is also clamped and off its food, it has become infected and needs treating in a hospital tank.