A fish that will not eat: the check order

Quick answer

A fish that suddenly refuses food is giving you information, and the order in which you check things matters. Water quality comes first, because ammonia and nitrite suppress appetite before they produce any other visible sign. Then temperature. Then the environment, meaning cover, light, flow and tank mates. Only then the food itself, and last of all disease.

Owners almost always run that list backwards. They change the food, then add a treatment, then finally test the water and discover the answer was in the first step. A new arrival that has never eaten is a slightly different problem again, and usually comes down to the fish not recognising what you are offering as food.

Target feeding with a pipette puts food where a specific fish is, which is often the whole problem in a busy community tank.

• Test ammonia, nitrite and temperature before you change anything else. Appetite is one of the first things water quality takes away.
• Food has to arrive in the right part of the tank at the right speed. A surface flake never reaches a bottom feeder in a competitive tank.
• Many nocturnal species simply do not feed while the lights are on.
• A newly imported or wild-caught fish may only recognise live or frozen food and needs weaning rather than persuading.
• A fish that is eating and still losing weight is a different problem from a fish that is not eating, and it points at internal parasites.

Decide in 60 seconds

The check order, and why it is that order

Water first.

Ammonia and nitrite are irritants that damage gill tissue and reduce appetite early. Nitrate at high levels does the same more slowly. A tank that has recently been overcleaned, had a filter replaced, been overstocked, or had a fish die unnoticed can go from stable to toxic quickly, and appetite is often the first thing you notice. Test rather than judging by how clear the water looks, because ammonia is invisible.

Temperature second.

Fish are ectothermic and their digestion runs at the temperature of the water. A heater that has failed, a room that has cooled, or a tank kept below the species range will produce a fish that eats little and digests slowly. Too warm is a problem in the other direction, because oxygen falls and metabolic demand rises.

Environment third.

A fish that feels exposed does not feed. New tanks with bright light and no planting, no shade and no cover produce fish that hide and refuse food. Strong flow across the surface scatters food before it can be taken. An aggressive tank mate, a new addition, or a fish being bullied out of the feeding area all produce refusal in one individual while everyone else eats.

Food fourth.

Size, sinking rate, texture and type. A pellet too large to fit in the mouth is refused. A flake that floats never reaches a bottom dweller. A dry pellet that expands after swallowing causes discomfort and then avoidance. Old food that has been open for months has lost both palatability and vitamins.

Disease last.

Internal parasites, bacterial infection and organ disease all reduce appetite, and they are real. They are simply not where you start, because the four earlier causes are far more common and much easier to correct.

Getting food to the right fish

This is the single most under-recognised cause of a fish that appears to be a poor eater in an otherwise healthy tank.

Fish occupy layers. Hatchetfish and many gouramis take food at the surface. Tetras, barbs and rasboras work the middle. Corydoras, loaches, most catfish and many plecos work the bottom. Food dropped in one place and eaten enthusiastically by fast midwater fish never arrives at the substrate.

Speed compounds it. Barbs and danios are quick, and a slow-moving fish with a small mouth simply loses every time.

The fixes are mechanical rather than nutritional.

Use sinking food designed to reach the bottom quickly, and put it in after the lights go off for nocturnal species.

Feed in more than one place at once so the fast fish cannot cover all of it.

Target feed with a pipette, a turkey baster or long feeding tongs, delivering food directly to a specific animal.

Use a feeding dish or a slate for bottom feeders so the food does not disappear into the substrate.

Feed algae grazers properly. Plecos and many other grazers need real wood to rasp and vegetable matter such as blanched courgette or a suitable wafer, and an aquarium that is spotlessly clean has nothing in it for them.

Old food loses palatability and vitamin content. Buy small tubs, keep them sealed and cool, and throw out anything opened long ago.

The new arrival that has never eaten

A fish that has not eaten since it entered your tank is a different case from one that has stopped eating after months.

Expect a settling period. Transport, netting, bagging, a change of water chemistry and a new environment all suppress appetite, and a fish that hides and refuses food for the first days is behaving normally. Add cover, keep the lights low, keep the room quiet, and offer small amounts rather than large ones.

Ask what it was eating. The most valuable question you can put to a shop, and the one almost nobody asks, is what this fish has been fed and whether it has been seen eating. Better still, ask to watch it eat before you buy.

Recognise the wild-caught problem. Many species in the trade are still wild-collected, and a fish that has only ever eaten live invertebrates does not perceive a dry pellet as food at all. Weaning is a ladder rather than a switch: start with live or frozen foods it accepts, then mix a small amount of prepared food into it, then increase the proportion over days to weeks. Soaking prepared food in the juice from thawed frozen food, or in a garlic-based preparation, is widely used to make it smell like something familiar.

Watch out for the species that genuinely cannot be weaned easily. Mandarin dragonets in marine tanks need a large established population of live copepods and starve slowly in a new system. Some anthias and small planktivores need very frequent small feeds. Freshwater puffers need hard-shelled food to control tooth growth. Buying these without a plan is where most of the losses happen, and the plan has to exist before the fish arrives.

Check the mouth. A fish that has been netted roughly or has spent a long time in a bag can have a damaged jaw, and one that repeatedly approaches food and fails to take it may not be refusing at all.

When it is disease

Two patterns point strongly at internal parasites, which are common in wild-caught fish and in newly imported stock.

Eating well and still losing weight. Nutrients are being taken by something else, or absorption is failing.

White stringy faeces trailing from the vent. This is intestinal lining being shed and is a recognised sign.

A third is more specific. Fine red thread-like structures protruding from the vent are Camallanus worms, which need a particular class of treatment rather than a general antiparasitic.

Other disease-related patterns worth knowing: a fish that takes food and spits it out repeatedly may have a mouth or gill problem rather than a preference; a fish that hangs at an odd angle and shows no interest may have a swim bladder or buoyancy problem making feeding uncomfortable; and a fish with a swollen abdomen and raised scales that has stopped eating is a very different and more urgent situation.

Assess condition properly rather than by impression. Look at the fish from above as well as from the side. A healthy fish is broad across the back and the shoulders. A fish with a narrow, knife-edged back seen from above is losing muscle even if the belly looks normal from the front.

Watch which fish actually take food, from where in the tank, and how fast. Two minutes of observation identifies most feeding problems.

What commonly goes wrong

Adding more food.

A tank where food is not being eaten does not need more of it. Uneaten food decays, ammonia rises, and the water gets worse for a fish that was already off colour.

Changing the food every day.

Fish need repeated exposure to recognise something as food. Switching daily means nothing is ever offered long enough to be learned.

Treating the tank blind.

Medication is a stressor and several products damage the filter bacteria. Dosing without a diagnosis frequently converts a feeding problem into a water quality emergency.

Assuming a fish is simply fussy.

Fish do not have preferences the way a dog does. Refusal has a cause, and in a fish that has been eating well it is usually environmental.

Feeding only in daylight.

Nocturnal species are widely believed to be poor eaters when in fact they have never been offered food at a time they are willing to come out.

Keeping food too long.

Fats oxidise and vitamins degrade in an opened tub. Buy small quantities, keep them sealed, cool and dark, and replace them regularly.

Overlooking bullying.

Aggression is often subtle and often happens when you are not watching. A fish that stays in a corner and does not approach the food may be being prevented from doing so.

What never to do

Do not add food to a tank with a detectable ammonia or nitrite reading.

Do not medicate a fish that has stopped eating without knowing what you are treating.

Do not raise the temperature to "encourage" feeding without checking the species range and increasing aeration.

Do not feed live foods from an unknown or wild source, since they can introduce parasites and disease.

Do not leave uneaten food in the tank to be found later.

Do not net and move a fish that is off food unless there is a specific reason, since handling adds stress and damages the slime coat.

Do not buy a species with a known specialist diet, such as a mandarin dragonet or an obligate planktivore, without having the food supply established first.

Do not assume a fish will eventually accept flake. Some will not, and a slowly starving fish looks normal for a surprisingly long time.

When to get help

Get advice the same day for a fish with a visibly pinched belly, a knife-edged back seen from above, red threads at the vent, a swollen abdomen with raised scales, or breathing difficulty alongside the loss of appetite.

Get advice within a few days for a fish that is eating and still losing condition, passing white stringy faeces, or that has not eaten at all since arriving despite the water, temperature and environment all being correct.

A settled fish that comes out for food, holds position and looks broad across the back is what you are aiming for.

Have ready the species, how long you have had it, what it was fed before you got it, the tank volume, the full water test results including temperature, what else lives in the tank and how they behave at feeding time, exactly what you have offered and when, and photographs of the fish from the side and from above. A vet or an aquatic specialist who sees fish can examine faeces and a skin scrape, which distinguishes internal parasites from the environmental causes quickly and stops you treating for the wrong thing.