Fish breathing hard when the water tests fine: gill disease

Quick answer

A fish that is working hard to breathe is telling you one of three things: there is not enough oxygen in the water, the gills cannot use the oxygen that is there, or the blood cannot carry it. Most owners test the water, find it acceptable, and stop. The second and third possibilities are where the diagnosis usually lives.

Increase surface agitation and aeration first, because that is safe in every one of the three scenarios and buys time. Then work out which one you are in, because the treatments have nothing in common.

A settled fish at rest, with both gill covers moving at the same steady rate, is your baseline. Learn it before you need it.

• Add aeration and surface movement immediately. It is never the wrong first step.
• One gill cover flaring more than the other points to a problem in that gill, not in the water.
• Gills held permanently open, or pale, patchy or brown gill tissue, is urgent.
• Nitrite poisoning stops the blood carrying oxygen even when the water is fully oxygenated.
• Labyrinth fish such as bettas and gouramis breathe air at the surface normally. Frequency changing is the signal, not the behaviour itself.

Decide in 60 seconds

How a gill actually works, and why that explains the sign

Water enters the mouth, passes over the gill arches, and exits under the operculum, the gill cover. Along each arch sit rows of filaments, and along each filament sit rows of thin plates called lamellae. Blood flows through the lamellae in the opposite direction to the water, which makes the exchange remarkably efficient.

Two things make it work: a very large total surface area, and a very short distance for oxygen to diffuse across.

Almost every gill disease attacks one or both.

Irritation, whether from ammonia, from parasites, or from suspended solids, causes the epithelium to thicken. Thicker means a longer diffusion distance. Chronic irritation causes the cells between lamellae to proliferate until adjacent lamellae fuse, which destroys surface area outright. Mucus produced in response coats the surface and adds another barrier. Bacterial infection erodes tissue and leaves the remainder inflamed.

The fish cannot change the anatomy, so it changes the flow. It ventilates faster and harder, which is exactly what a fish in low-oxygen water does. That is why you cannot tell these apart by watching the breathing alone, and why the water test result does not settle the question.

There is a third possibility that has nothing to do with either. If the blood itself cannot carry oxygen, the gills and the water can both be perfect and the fish still suffocates. Nitrite does this by converting haemoglobin into methaemoglobin, which does not release oxygen to the tissues. The gills go brown, the fish gasps, and adding an airstone does not help.

Learning the baseline before you need it

Count opercular beats. Choose a fish, watch one gill cover, and count the movements over a fixed period using a timer. Do it when the fish is at rest, not after you have chased it with a net or just fed it, and do it at the same time of day.

Rates differ enormously between species, between individuals, and with temperature, which is why there is no universal normal number to quote. What you want is your fish's number in your tank.

Record it for each of your main fish once a month. When something changes, you will have an objective comparison instead of an impression.

Also learn what the gills look like. A brief look at a healthy fish, viewed from the side with good light as the gill cover opens, shows red, evenly coloured tissue. Pale pink, patchy, brown, grey or ragged tissue is abnormal, and knowing the healthy version is what lets you recognise it.

Gill flukes and other parasites

Monogenean flukes are the commonest parasitic cause of gill problems in aquarium fish. They are small flatworms that attach to the gill tissue with hooks and feed on it, and they are entirely invisible to the naked eye.

The important practical points are these.

They have a direct life cycle, so they do not need an intermediate host and they multiply within your tank.

Some genera lay eggs and some give birth to live young. Egg-laying species matter for treatment, because the eggs are not killed by the medications used on the adults, and they hatch over a period that depends on temperature. Treatment therefore has to be repeated at an interval that catches each hatch, and a single dose almost always fails.

Diagnosis is by taking a small sample of gill tissue or mucus and examining it under a microscope. That is not something you can do by looking at the fish, and it is the reason aquatic veterinary input changes outcomes so much in this problem.

Other gill parasites include ciliates such as those that cause white spot, which have a gill phase before you ever see spots on the body, and velvet, which coats the gills before the dusty appearance is visible on the skin. Larger parasitic copepods are visible on the gill filaments in pond fish.

The pattern that suggests parasites is several fish affected, flashing and rubbing, clamped fins, and rapid breathing that arrived within days to weeks of adding new fish or plants without quarantine.

Bacterial gill disease and physical damage

Bacterial gill disease usually follows something else: crowding, high organic load, poor flow, chilling or a water quality problem. The gill filaments become swollen, clubbed and covered in mucus, and secondary fungal growth may follow.

Columnaris can affect gills as well as skin, and gill involvement is the more dangerous form because it progresses fast and the external signs are minimal.

Physical and chemical damage matters longer than owners expect. A fish that survived an ammonia spike, a nitrite spike, a chlorine exposure from unconditioned tap water, or a chemical accident has damaged gill epithelium, and that tissue takes weeks to remodel. During that time the fish has less functional gill than it should and will breathe harder than its tank mates. The management is patience: pristine water, high oxygen, light feeding, and no additional stressors.

The water-side causes worth knowing properly

Low dissolved oxygen. Driven by temperature, stocking density, organic load, lack of surface movement, and at night by plants and by the biological filter consuming oxygen. The classic picture is every fish at the surface at dawn in summer.

High carbon dioxide. In heavily planted tanks with carbon dioxide injection, or in an overstocked tank with poor gas exchange, carbon dioxide accumulates and makes it harder for fish to unload it, which produces laboured breathing even when oxygen is adequate.

Nitrite. As described. Test it whenever a fish is breathing hard. Chloride is protective, which is why aquarists add salt in nitrite poisoning, but salt is not appropriate for every species or every tank so check before you reach for it.

Ammonia. Directly damages gills as well as being toxic systemically.

Chlorine and chloramine. From tap water added without conditioner, or from a conditioner dosed for the wrong volume. Gills are the first tissue exposed.

Gas supersaturation. Less familiar and worth recognising. If a pump draws in a small amount of air on its suction side, or if a tank is topped up rapidly with cold water straight from a pressurised mains supply, dissolved gases exceed saturation. Bubbles then form in the tissues, including in the gill capillaries. The tell is tiny bubbles in the fins, in the cornea, or under the skin, alongside gasping. Vigorous aeration and finding the leak are the answers.

Temperature. Warm water holds less oxygen and raises demand. A heater stuck on, a hot room, or a summer pond in still weather all produce it.

What to do, in order

Increase aeration and surface agitation now. An airstone, an additional powerhead angled at the surface, or the spray bar raised to break the surface. This helps in low oxygen, in carbon dioxide accumulation, in supersaturation and in gill disease, and it does no harm in any of them.

Stop feeding for the moment. Digestion increases oxygen demand and uneaten food increases the organic load.

Test the water and write the numbers down. Ammonia, nitrite, nitrate, pH, temperature. If nitrite is present, act on it immediately.

Do a water change with dechlorinated, temperature-matched water.

Look at the fish properly. Both gill covers, from both sides, in good light. Note whether one side is different, whether the gills are held open, and what colour the tissue is.

Count the rate, and count it again in an hour, so you know which direction it is going.

Check the equipment. Heater reading, pump for air leaks, filter flow, and whether anything was changed in the last week.

Then seek a diagnosis rather than a product. A gill sample under a microscope answers in minutes what weeks of general remedies will not. Many aquatic veterinary services will look at a sample, and some will advise remotely with photographs and your written water results.

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Do not treat blind with a sequence of remedies in a tank where fish are struggling to breathe.

Many aquarium medications reduce dissolved oxygen, some are directly harmful to gill tissue, and several are dangerous to scaleless species such as loaches and to invertebrates. Stacking them on a fish with compromised gills is a common way to lose the whole tank. If you must treat before you have a diagnosis, treat one thing, with heavy aeration, in a hospital tank of known volume, and remove carbon from the filter first because it will otherwise strip the medication out.
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What changes by species

Labyrinth fish. Bettas, gouramis and paradise fish have an accessory air-breathing organ and go to the surface for air as normal behaviour. A betta surfacing is not a red flag. A betta surfacing far more often than it used to, or hanging at the surface with clamped fins, is.

Loaches and some catfish. Several gulp air from the surface and pass it through the gut. Again, the change in frequency is the signal.

High-demand species. Fast-swimming open-water fish, many danios, most loaches, and cold-water species in warm water show oxygen problems first.

Deep-bodied fancy goldfish. Poor swimmers with compressed body cavities, and often the first to show distress in a tank with marginal oxygen.

Bottom dwellers. Oxygen is lowest near the substrate in a poorly circulated tank, so corydoras and loaches may be affected while the mid-water fish look fine.

Pond fish in summer. Warm still water, algae consuming oxygen overnight, and a heavy stocking of koi is the classic combination for a dawn emergency.

What never to do

Do not raise the temperature to help a fish that is breathing hard, unless a vet has directed it and you have added heavy aeration.

Do not add multiple medications in sequence without full water changes and a reason.

Do not add salt without checking it is safe for every species and plant in the tank.

Do not net and move a struggling fish unless you have somewhere better prepared. Handling costs oxygen.

Do not feed heavily while a fish is compromised.

Do not clean all the filter media at once. Losing the biological filter turns a breathing problem into an ammonia crisis.

Do not top up rapidly with cold water straight from a pressurised tap.

Do not assume that acceptable ammonia and nitrite readings rule out a gill problem. They rule out one of three groups.

When to get help

Immediately, on the water rather than the fish, for any detectable ammonia or nitrite, and for a whole tank at the surface.

Urgently, from an aquatic veterinarian or a fish health service, for gills held permanently open, for pale, brown or ragged gill tissue, for asymmetric gill movement, and for several fish deteriorating together.

Promptly for rapid breathing with flashing and clamped fins, which suggests a parasite burden that will not clear on its own.

Book a considered conversation for a single fish that breathes slightly faster than its tank mates and is otherwise well, and use it to review stocking, flow, surface movement and quarantine practice.

Bring your written water results, tank volume, full stock list, the date and source of the last addition, filtration and flow details, the video of the fish, and any products already added. In gill disease the answer is usually a combination of the water history and what is actually on the gill, and only one of those can be seen from outside the glass.