My fish is breathing fast at the surface: is it a disease?

Very often no, at least not in the sense of an infection. Gasping at the surface and fast breathing point in most cases to a water problem: low oxygen, temperature too high, or the presence of ammonia and nitrite, which are toxic. The first move is not a medicine, it is to measure the parameters and immediately do a partial water change with conditioned water at the same temperature, also increasing surface movement to oxygenate. If the sign remains after the water is sorted, then it is worth looking further with a vet experienced in fish.

Can I pour medicines straight into the main tank?

It is best avoided as a first move. Many treatments damage the biological filter, the bacteria that keep the water clean, and also stress healthy fish and plants. Pouring medicines across the board at the first suspicion risks creating a bigger problem than the one you want to solve. The better route is to correct the water, isolate the sick fish in a quarantine tank and treat there, in a targeted and controlled way. The whole main tank is treated only in specific cases and when the nature of the problem really requires it.

How long should quarantine last for a new fish?

A couple of weeks is the period many fishkeepers consider reasonable to let any disease surface before introducing the fish into the community. In that time you observe the newcomer in a separate environment, check that it eats and swims normally and confirm no signs of illness appear. Quarantining new arrivals is the single precaution that prevents the most outbreaks in a tank, because the most common way diseases enter is precisely a fish introduced without a check.

When should I turn to a vet rather than handle it myself?

When the signs are serious or do not improve after you have sorted the water and isolated the fish. A swollen abdomen with scales raised like a pine cone, several fish affected at once, a sudden die off, or symptoms worsening rapidly despite the basic steps are situations beyond do it yourself. Not all vets treat fish, so it is worth identifying a professional experienced with exotic and aquatic animals in advance, so you do not have to search in the moment of emergency, when every hour counts.

Non conventional species35 minLevel: IntermediateUpdated on August 20, 2026

Recognising and managing common aquarium fish diseases

White spots, ragged fins, laboured breathing: how to spot the most common aquarium fish diseases and why the cure almost always starts with the water.

Audience: Pet owners
Species: Fish
Scope: Valid everywhere, Italy, European Union

What you need before you start

Un test dei parametri dell'acqua affidabile per ammoniaca, nitriti, nitrati e pH, meglio a reagenti liquidi che a striscette
Un acquario di quarantena già pronto o allestibile in poche ore, con filtro maturo e riscaldatore
Il condizionatore d'acqua, un secchio dedicato e il numero di un veterinario esperto di pesci per i casi dubbi

When a fish falls ill, the instinct is to rush out and buy a medicine, but that is almost always the wrong order of things. The vast majority of aquarium fish diseases do not come from a germ that happened along, they come from an environment that has stopped working: water with too much ammonia or nitrite, unstable temperature, overcrowding, swings that weaken the defences. A sick fish is often the warning light of a sick tank. That is why the first move in front of a struggling fish is not to open a bottle, it is to measure the water parameters. This guide helps you observe the right signs, recognise the most common diseases without claiming to run a laboratory diagnosis, use the quarantine tank as your main tool, and understand when the situation is beyond do it yourself and needs a vet experienced with aquatic animals.

First of all, look at the water

A fish lives immersed in its environment around the clock, so water quality is its health, not a side detail. Before guessing at any disease, measure the parameters: if ammonia or nitrite are above zero, you have already found the most likely cause of the trouble, and no medicine will work until the water is put right. Fixing the environment is often half the cure.

Indicative reference parameters for a freshwater community tank. Guide values only: needs change from species to species.
Parameter	Expected value	What an altered value points to
Ammonia	0	Immature or overloaded filter, immediate danger
Nitrite	0	Nitrogen cycle incomplete, toxic to fish
Nitrate	Low	Water changes too rare or too much food
Temperature	Stable per species	Swings that weaken the defences
pH	Stable per species	Swings more harmful than the value itself

Observing the right signs

A fish cannot tell you it is unwell, but it changes behaviour and appearance well before reaching a crisis. Learning to look every day, perhaps at feeding time, lets you catch the signs while there is still time to act. A healthy fish swims smoothly, eats eagerly, has intact fins and full colours, and breathes steadily.

Fast breathing at the surface or gasping: often signals low oxygen or poisoned water, not a contagious disease.
A fish rubbing against decor and substrate: itching from external parasites or irritating water.
Ragged, reddened fins with whitish edges: typical of bacterial infections favoured by poor water.
White spots like grains of salt on body and fins: a picture consistent with white spot disease.
Unbalanced swimming, a fish floating or sinking without control: a possible swim bladder problem.
Loss of appetite, dull colours, hiding in a corner: non specific signs that still deserve a water check.

The most common diseases and what to do

Some pictures recur so often that they are worth recognising, without turning yourself into a diagnostician. The aim is not to put an exact name to every germ, it is to grasp the category of problem and act the right way, which always starts with improving the water and often runs through quarantine. Faced with doubt or a rapid worsening, a vet experienced with fish remains the reference.

White spot disease: small white grains scattered over the body, the parasite has a temperature linked cycle and is managed with dedicated treatments and an optimal environment.
Fin rot: frayed, whitish edges that advance, almost always secondary to poor water, improving by correcting the environment and with specific care in advanced cases.
Fungal infections: cottony white tufts on wounds or mouth, striking fish already weakened and stressed.
Swim bladder problems: unbalanced swimming, often linked to overfeeding or the wrong food rather than an infection.
Dropsy: a swollen abdomen and raised scales like a pine cone, a serious sign of internal suffering that calls for the vet.

Using the quarantine tank

The quarantine tank is the most underrated and most useful tool in fishkeeping. It serves at two moments, when a new fish arrives and when one falls ill: in both cases it separates the problem from the rest of the community and lets you treat in a controlled environment, without poisoning the main tank's biological filter with medicines. Here is how to run it.

Prepare a simple, safe tank
All you need is a clean container, a heater, a mature filter or an already colonised sponge, and a lid. No elaborate substrate: it must be easy to clean and to observe.
Move the fish gently
Transfer it with a container, not with a net that damages the protective slime, and use water from the source tank to reduce shock. The temperature must match.
Keep the water perfect during treatment
In quarantine the parameters matter even more: make small frequent water changes, because the small volume fouls quickly and stability is part of the therapy.
Treat only what is needed and as directed
If you use a product, follow the dose and duration exactly, without stopping at the first improvement. A half finished treatment selects the more resistant germs and makes the problem harder.
Return the fish only once recovery is confirmed
Wait for the fish to eat, swim and look normal for several days before putting it back with the others, reacclimatising it calmly to the temperature and water of the main tank.

Prevention is worth more than any cure

Treating a tank that keeps falling ill is a losing battle: if diseases keep coming back, the problem is the environment, not bad luck. Prevention in fish coincides almost entirely with good tank management, and it is made of simple, steady habits that keep the immune system of every occupant high.

Make regular partial water changes and measure the parameters consistently, without waiting for something to go wrong.
Do not overcrowd the tank and respect each species' need for space and company.
Feed in moderation, because excess food is the leading cause of spoiled water and digestive problems.
Keep temperature and pH stable, avoiding the swings that weaken the defences more than a slightly different value.
Quarantine every new arrival and observe the fish every day, so you catch problems while they are still small.

In an aquarium you do not treat the fish, you treat the water: when the environment is right, diseases become rare and the fish defend themselves.
Principle of responsible fishkeeping

Frequently asked questions

My fish is breathing fast at the surface: is it a disease?: Very often no, at least not in the sense of an infection. Gasping at the surface and fast breathing point in most cases to a water problem: low oxygen, temperature too high, or the presence of ammonia and nitrite, which are toxic. The first move is not a medicine, it is to measure the parameters and immediately do a partial water change with conditioned water at the same temperature, also increasing surface movement to oxygenate. If the sign remains after the water is sorted, then it is worth looking further with a vet experienced in fish.
Can I pour medicines straight into the main tank?: It is best avoided as a first move. Many treatments damage the biological filter, the bacteria that keep the water clean, and also stress healthy fish and plants. Pouring medicines across the board at the first suspicion risks creating a bigger problem than the one you want to solve. The better route is to correct the water, isolate the sick fish in a quarantine tank and treat there, in a targeted and controlled way. The whole main tank is treated only in specific cases and when the nature of the problem really requires it.
How long should quarantine last for a new fish?: A couple of weeks is the period many fishkeepers consider reasonable to let any disease surface before introducing the fish into the community. In that time you observe the newcomer in a separate environment, check that it eats and swims normally and confirm no signs of illness appear. Quarantining new arrivals is the single precaution that prevents the most outbreaks in a tank, because the most common way diseases enter is precisely a fish introduced without a check.
When should I turn to a vet rather than handle it myself?: When the signs are serious or do not improve after you have sorted the water and isolated the fish. A swollen abdomen with scales raised like a pine cone, several fish affected at once, a sudden die off, or symptoms worsening rapidly despite the basic steps are situations beyond do it yourself. Not all vets treat fish, so it is worth identifying a professional experienced with exotic and aquatic animals in advance, so you do not have to search in the moment of emergency, when every hour counts.

What to do next

When a fish is unwell, always measure the water parameters first and correct ammonia, nitrite and temperature with a partial change, because the environment is half the cure. Observe the fish every day at feeding time, recognise the category of problem without claiming a laboratory diagnosis, use the quarantine tank to treat the sick and to receive new arrivals, and turn to a vet experienced in fish when the signs are serious or fail to improve.