Fish: obesity, fatty liver, and what a swollen fish actually means

Quick answer

A fat fish and a swollen fish look similar from the front of the tank and mean completely different things. Fat accumulates slowly, symmetrically, and mostly across the back and shoulders. Fluid accumulates faster, pushes the belly out and often lifts the scales. One is a feeding problem you can correct over months; the other is usually organ failure and is urgent.

Obesity in ornamental fish is real, common and under-recognised, because there is no scale to weigh them on and no obvious ribs to feel. It is driven by prepared foods that are far denser than anything the species evolved to eat, fed several times a day into a tank where the fish has almost nowhere to swim.

Measuring the food, rather than shaking the pot, is the whole intervention for most overfed fish.

• Look at your fish from directly above. Fat shows across the back; fluid shows as a rounded belly and raised scales.
• Raised scales that give a pinecone outline are not obesity. That is a same-day problem.
• Do not fix an overweight fish by starving it. Fat mobilised too fast overloads a liver that is already fatty.
• Herbivorous fish fed protein-rich food do not simply get fat, they get sick.
• Feeder fish are a poor diet for predatory fish and carry a specific vitamin problem.

Decide in 60 seconds

How fish become obese

In the wild, most ornamental species spend a large part of the day working for a low-energy diet. Goldfish and koi are cyprinids without a true stomach: food passes more or less continuously through a long intestine, and the animal is built to graze on plant matter, detritus and small invertebrates. A pellet made from fish meal and grain delivers, in one mouthful, more energy than that system evolved to receive in a much longer period.

At the same time, activity collapses. A fish in a tank that gives it a few body lengths to travel expends a fraction of the energy of a fish that covers open water.

Excess energy in fish is stored as lipid, and it goes to two main places: the mesenteric fat that surrounds the organs, and the liver itself. Fat inside liver cells displaces functional tissue, and a fish with hepatic lipidosis has reduced capacity to process what it eats, reduced immune function and reduced resilience to any other stress. It is one of the most common findings when long-kept ornamental fish are examined after death.

Because none of this is visible, the fish looks normal until either the body shape changes or something else goes wrong that a healthy animal would have survived.

Reading the body shape properly

Look from directly above.

This is the view that shows fat, and almost nobody uses it. Stand over the tank or pond, look down the length of the fish, and assess how broad it is across the shoulders and back relative to the head.

A fish in good condition tapers smoothly from behind the head. An overweight fish is broad and blocky across the back, and the widest point is well behind the gills.

Look from the side for fluid.

Ascites, the accumulation of fluid in the body cavity, pushes the abdomen down and out. It is generally faster in onset than fat and is usually accompanied by other signs.

Look at the scales.

This is the crucial one. Scales that stand out from the body so that the fish looks like a pinecone from above mean fluid is accumulating under the skin. This is not a fat fish. It is a fish whose osmoregulation, kidneys or circulation are failing, and it is a serious sign.

Compare the sexes and the season.

A gravid female swells symmetrically in the lower abdomen behind the pelvic fins, remains active, keeps eating, and returns to normal after spawning. In many species males develop breeding tubercles or colour changes at the same time, which helps confirm what you are looking at.

Photograph it.

Once a month, from directly above and from the side, at the same distance, against the same background. Fish change slowly enough that memory is useless and photographs are conclusive.

Energy density is invisible in a pellet. A small volume of a high-fat sinking pellet delivers far more than the same volume of a vegetable-based flake.

The species differences that change everything

Goldfish and koi.

No stomach, long intestine, built for grazing. They are prone to becoming both overweight and constipated on high-protein pellets, and fancy varieties whose body shape is already compressed suffer buoyancy problems when visceral fat presses on the swim bladder. A diet with a substantial vegetable component, and food that sinks rather than floats for varieties prone to gulping air, both help.

Bettas.

Tiny stomach and an enthusiastic feeding response. Overfeeding and constipation are among the most common problems in the species. A small number of appropriately sized pellets is a meal, and the fish will beg regardless.

African rift lake cichlids and other herbivores.

These are the species where diet is not just a weight question. Herbivorous cichlids fed on high-protein foods, including beef heart and protein-heavy pellets, develop serious intestinal problems. The presentation is bloating, loss of appetite, white stringy faeces and rapid deterioration, and it responds badly once established. Feed the vegetable-based food the species requires from the start.

Plecos and other algae grazers.

Many need fibre and, in the case of wood-eating species, actual wood in the tank. A pleco fed only on protein wafers is not being fed correctly however round it looks.

Predatory fish such as oscars and arowana.

Two problems arrive together. Feeder fish are a fatty, poorly balanced diet, and many common freshwater feeders contain thiaminase, an enzyme that destroys vitamin B1. A predator fed exclusively on them can develop both fatty liver and thiamine deficiency, which causes neurological signs. Good quality prepared foods and appropriate whole frozen items are a better basis.

Marine fish.

Requirements differ widely between herbivorous tangs, planktivores and predators, and the same principle applies: the food has to match the species, not the tank.

Reversing it without causing harm

Do not starve the fish.

This is the most important line in the topic. Rapidly mobilising stored fat in an animal whose liver is already fatty can make liver function worse. Weight reduction in fish should be gradual, over months, achieved by feeding less and better rather than by not feeding.

Reduce the amount before you reduce the frequency.

Cutting each feed while keeping the routine keeps the fish's behaviour stable and reduces the begging that drives owners back to overfeeding.

Portion deliberately.

Stop shaking the pot over the tank. Measure a portion out into your hand or a small container first, every time. Feeding by what disappears in a couple of minutes is unreliable, because a hungry fish will eat far more than it needs in that window.

Change what is in the portion.

For grazers and herbivores, move toward vegetable-based foods, blanched vegetables such as courgette, peas or spinach, and spirulina-based products. Reduce high-fat items such as freeze-dried and frozen bloodworm to occasional use.

Introduce fasting days if they suit the species.

Weekly fasting is standard practice for goldfish and koi keepers and is well tolerated. It is not appropriate for every species, particularly small fast-metabolism fish and young growing animals.

Make the fish work.

Scatter sinking food across the tank rather than dropping it in one place. Clip vegetables to the glass so they have to be grazed. Increase flow if the species tolerates it. Use the largest tank the species and your space allow, because swimming distance is the only exercise variable you control.

Adjust for temperature.

A fish kept at the lower end of its range needs less food than the same fish kept warm. Pond fish need their feeding reduced as autumn cools the water and stopped altogether when it gets cold.

Monitor by photograph.

Monthly overhead photographs at the same distance are the only practical way to track progress. Expect it to take months and to be barely visible between shots.

Where the usual advice goes wrong

"Feed as much as they can eat in two minutes, three times a day."

This is the single most repeated piece of aquarium advice and it is a recipe for overfeeding. Fish will eat well past what they need, particularly cyprinids with no stomach to signal fullness. It also loads the tank with waste.

"Just fast it for a week and it will slim down."

Prolonged fasting to reduce a fat fish risks worsening hepatic lipidosis. A short fast is a reasonable tool for constipation. It is not a weight loss programme.

"Feed peas for anything bloated."

Deshelled cooked peas are genuinely useful for constipation in goldfish and some other species, and they are the wrong answer entirely for dropsy, an internal mass, or bacterial bloat in a herbivorous cichlid. Decide what you are looking at first.

"A bigger fish is a healthier fish."

Owners frequently take a broad, deep-bodied goldfish as a sign of good keeping. Length reflects growth; width across the back reflects fat.

"It is just a goldfish, a bowl is fine."

A small unfiltered container is the classic setting for this problem: no room to swim, no filtration to handle the waste from overfeeding, and every scrap of food within reach of a fish that cannot move away from it.

This is the setup to avoid. No filtration, no swimming distance and food that never gets out of reach make overfeeding almost inevitable.

What the swelling is, if it is not fat

Dropsy. Fluid accumulating under the scales and in the body cavity, producing the pinecone appearance. It is a sign rather than a disease, and the underlying cause is usually kidney failure, severe bacterial infection or organ failure. Prognosis is guarded and it needs prompt advice.

Gravid female. Swollen behind the pelvic fins, symmetrical, active, eating, seasonal, and resolves after spawning.

Egg binding. A female that has been swollen for a long period without spawning, particularly where there is no male or no suitable spawning site.

Internal mass. Asymmetric swelling, sometimes visible as a distinct bulge on one side, developing over weeks to months. Common in older fish.

Constipation. Bloating with reduced or absent faeces, or long trailing stringy faeces, usually in a fish on a dry high-protein diet with no fibre.

Bacterial bloat in herbivorous cichlids. Distension, loss of appetite, white stringy faeces, rapid decline. Diet and stress related, and it needs prompt treatment.

Swim bladder disorder. Not swelling at all in some cases, but a fish that floats, sinks or rolls. In fancy goldfish it is frequently linked to gut distension and to visceral fat.

What never to do

Do not starve an overweight fish to slim it down.

Do not feed a pond in cold water.

Do not feed a herbivorous cichlid on protein-rich foods, beef heart or general-purpose pellets.

Do not use live feeder fish as a staple diet for predatory species.

Do not treat pineconing as obesity.

Do not add medication to a whole tank because one fish looks swollen. Identify the problem first, and remember that many treatments harm invertebrates, plants and the filter bacteria.

Do not increase feeding because a fish begs. Begging in fish is a learned response to a person approaching, not a hunger signal.

Do not change several things at once. Alter the diet, then wait, then reassess, or you will not know what worked.

When to get help

Seek advice from an aquatic vet or an experienced fish specialist the same day for raised scales, bulging eyes, red streaking in the fins or body, gasping at the surface, or a fish that has become swollen quickly.

Seek advice promptly for a herbivorous cichlid that is bloated and off food, for a fish producing long white stringy faeces, and for any swelling that is asymmetric or is a discrete lump.

Book a review of the diet if a fish is broad across the back, if buoyancy problems keep recurring after meals, or if you have several fish in the same tank all becoming deep-bodied.

The target is a fish that tapers behind the head, moves easily through open water and works a little for its food.

Bring the species and how long you have kept it, tank or pond volume, current water test results for ammonia, nitrite, nitrate and pH, the water temperature, the exact food and how much and how often it is fed and by how many people, any recent changes, whether other fish are affected, and the monthly overhead photographs. In fish, the water is part of the history, and a swelling problem is frequently a water quality problem wearing a different hat.