Do fish know when another fish is dying?

Table of Contents

A lot of animals will not know when one of theirs is about to die, and the possibility of some species knowing is extremely rare. So, where do fishes stand in this case? Does a fish know when another is dying? Are they capable of such emotion?

Fishes are no doubt beautiful creatures and great pets if you ever care to get close to them. These cold-blooded animals have their way of life and laws guiding their cohabitation. Like most animals, they understand boundaries, know when to back out of confrontations, and can certainly identify the predators among their folds.

These abilities can be considered emotions or instinctive behaviours, which can make fish owners feel fishes are also capable of more complicated emotions; Like knowing or at least sensing when one of them is about to die.

However, there is no real research that supports this hypothesis. Fishes have never been scientifically confirmed of having insights or understanding when one of theirs is ill and about to die. They may be able to understand basic urges like eating and hiding from danger, but expressing complicated emotions has never been associated with them.

Several fish owners claim that their fishes have been able to exhibit behaviours similar to that emotion. An owner once stated that when one of his fish was ill and dying, other fishes took it upon themselves to bring food to it.

However, while this claim may be true, it does not in any way specify that the fishes knew the incapable one was dying. Fishes do not know when another is dying from research and observations currently available. As time goes on, more technological advancements will be made, and further research will be carried out that may change our stance.

Do fish know when another fish is dying

Do Fish Mourn Fish?

Fishes are barely capable of expressing emotions, according to research. While there is detailed research on fish psychology, none specifically states that they can express emotions. Hence stating that fishes have the ability or capability to mourn will be unfounded. There have been claims that fishes cared for at home have, at a point in time, shown the ability to express grief, but these claims are extremely doubtful as there are no scientific metrics to measure this emotion in fishes.

Healthy fish are happy fish.  Click this affiliate link to give your fish a healthy advantage.

The answer on fish ability to mourn has been a bit vague before proper research came into the limelight. Even with tons of research, the answer to this still seems unclear as a lot of professionals and vets are quite divided on a conclusion.

A large number of reports on grief emotion in fishes have come from tank and aquarium owners who claim that they have observed a behavior similar to mourning from their swimming pets after losing a tank mate. Some vets and professional aquarists also agree with these claims.

A tank owner once claimed that his goldfish showed grieving emotions. He explained that he had a male and female goldfish who spent a lot of time together as playmates. After several years one of them died. After the death, the other immediately became inactive and would always go to the spot where the other fish had lain dead. This continued for weeks before it became active again.

Click this affiliate link to order a special treat for your fish.

This is simply one of several claims of tank owners stating how their fish were able to express grief. As it stands, the answer is still not clear, but with available information and objective research, fishes do not possess empathy or emotion to grieve for their own.

How Do Fish Act When They Are Dying?            

When drawing towards death, a fish will definitely show signs indicating that all is not well. At this point, if you do not attend to it, death will be inevitable. Hence paying close attention to your fish can help you attend to it on time.


Loss of Appetite

This is a definite sign that all is not well with the fish. Fishes, when healthy, have a voracious appetite for their size and would normally make an effort to eat. 

Rejecting food is a telltale sign that your fish is in pain or stress.

Fighting for oxygen                                                                

A high presence of ammonia and nitrate in water leads to low oxygen availability, making it impossible for the fish to breathe and thrive. When this happens, the fish fights its way to the surface for oxygen. This exercise, if continued for long, wears down the fish and makes it susceptible to disease. This, coupled with the lack of oxygen, will eventually kill the fish.


Change in behavioural pattern                   

Fish have certain characteristics, with each species displaying a unique behaviour. Some fish prefer to rest in the day and become active at night. If a fish is unhealthy and about to die, a reversal of behaviour is likely to happen due to a lack of coordination.


Disturbing swimming pattern                               

When your fish continually and purposely hits things inside the Aquarium, it is a sign that it is stressed and desperate for relief. You should immediately consult with a vet, as early diagnosis increases its chance of survival.

Abnormal Speed in Breathing

One of the signs of a dying fish is its inability to breathe normally. When ill, fishes may have difficulty getting oxygen into their cells, triggering extremely fast air intake to meet up with the required oxygen for sustenance.         

Click this affiliate link to order a special treat for your fish.


Why Are My Fish Dying All Of A Sudden?

Are your fish suddenly dying in their numbers, and you don’t know why it is happening? Then this is for you. It is not new for fishes to suddenly drop dead for supposedly no reason. We shall highlight some possible reasons for the death of your fishes.


Stress is one of the most common causes of death in fish, and while you may not be informed of its symptoms, they are quite obvious. We will be highlighting them so you will be able to act quickly when next they pop up.

Glass surfing is one of the apparent symptoms of stress. It occurs when your fish frantically and consistently swims up and down in the tank. It is a telltale sign that the fish will rather be somewhere else than in there. This could be as a result of unhealthy water or a small aquarium.

Another sign of stress is excessive hiding by the fish. This could be caused by a bully in the tank or other reasons. Whatever it is, it is a bad sign, especially when your fish’s natural characteristics doesn’t include hiding. 

Unhealthy water

This can lead to fishes’ sudden death as water high in nitrate and ammonium is unsuitable for fishes due to its low oxygen. The lack of oxygen would force the fish to outsource on the water surface. Fishes cannot hold on for long like this and will eventually wear out and die.

Changing the water too frequently

Frequent water change can also lead to fishes’ sudden death as it unbalances the chemistry formed by the fishes with their surroundings. This usually leads to shock and eventual death.

Toxic materials

Materials like body creams, beverages, and detergents, can prove fatal if they somehow get into your tank. Your once healthy fishes could all be wiped out as a result of these materials. If these toxic items get into your tank, you must immediately drain it and call a vet.

Healthy fish are happy fish.  Click this affiliate link to give your fish a healthy advantage.

What To Do When A Fish Dies In An Aquarium      

When a fish dies in an aquarium, it is essential that an immediate evacuation be carried out to keep the other fishes safe. We will be providing tips on how to keep the aquarium safe.

The first step is to remove the fish from the aquarium. This is very important because the carcass can easily decompose due to the high presence of bacteria in the water. Decomposition in the aquarium will lead to extreme health risks for other fishes due to contamination of water. Also, not removing the carcass leaves other fishes vulnerable to a possible killer disease by feasting on it.

After removal, a thorough examination should be carried out on the fish. Were the fins pulled off or scuffed? This can help hint if the fish was killed in a brawl by a tank mate. However, if there is no such scuffle and the fish seems bloated, then it was likely killed by a disease.

If the fish seems bloated or does not have any sign of brawl marks on it, then the cause of death is clearly an internal cause. With the cause confirmed, the next step will be to carry out a test of the water. Check for the concentration of nitrate and ammonium to know if it is overboard. If everything seems fine, carry out a close observation for a couple of days to see if another fish behaves strangely. If a strange behaviour is noticed, contact a vet immediately.

For the dead fish, the next step will be to dispose of it. You should not attempt to flush it as it could lead to pipe blockage. To carry out proper disposal, you can dig a small grave and bury it or carry out a full cremation.

Click this affiliate link to order a special treat for your fish.

What To Do When Your Fish Is Dying?

When your fishes are dying, and you don’t know the cause, it could seem frustrating and sad. However, there are many things you could do to stop this unfortunate situation. We shall highlight some steps to help you stop the deaths. If there is still no positive result after carrying out these steps, you should contact a vet.

Test the water

One of the major causes of fish death is unhealthy water. Water highly concentrated with ammonium and nitrate (caused mostly by new tanks) can be very harmful and fatal to fishes. If your fish is dying, you should test the water, and if high in ammonium and nitrate, change it. You will have to carry this out as regularly as is needed to get the tank healthy.

Clean their Gills

Fish can only survive if they can breathe; hence any blockage impairing their oxygen access must be thoroughly cleaned. Your fish can get their gills stuffy or blocked, and you will have to clear them to allow normal breathing again. 

To clean, hold your fish while in the tank and take off the dirt.

Treat with Salt Water

Treating fish with saltwater can effectively stop the death toll if the cause is illness-related. There are two types of saltwater treatment that you can administer to help keep your fish healthy. They are the aquarium saltwater and Epsom saltwater treatments.

Epsom salt water treatment helps eject excess waste from your fish, making it feel lighter and healthier.

Aquarium saltwater treatment helps treat the parasitic ailments and rot in fins.

To perform any of these treatments, mix a tablespoon of salt with a gallon of water and place the fish inside for 3 mins before returning to the tank.

Click this affiliate link to order a special treat for your fish.

What To Do With A Dead Fish

There are a lot of reasons to quickly do away with a dead fish. From the possibility of a transfer of the bacteria responsible for its death to other fishes and the stench, a dead fish should not be around the neighborhood. Here is how to handle a dead fish.

Remove the dead fish

After discovering that a fish is dead, immediately remove it from the aquarium before it starts to rot due to the bacteria-filled water. This is also necessary as the corpse if left for too long, will risk the health of other fish due to pollution.

If its cause of death is from a disease, the delay could lead to other fishes consuming it and risk contracting it. Hence immediate removal is advised.

Examine it

If the time of death and finding the corpse is short enough, you can run a quick diagnostic check to see if you can pick up any hint on the cause of its death. Are the fins still intact or shredded to suggest bullying or a fight, or is the body bloated to suggest that the fish had died from an infection or disease?

Test the water

The water should immediately be tested after evacuating the fish to know if the cause of death was due to high ammonium and nitrate concentration. You should also test for nitrite and PH.

Unhealthy water is one of the highest causes of mortality in fishes, so this phase should not be ignored. 


You can get rid of the fish safely by burning it as the ashes cannot carry infections.


This is the best preferred method for disposing of a dead fish. A 3 feet deep hole is advised so other domestic animals can’t dig out the buried fish easily when moving around.

Fish Keep Dying, But Water Is Fine.

There are a lot of reasons that could lead to the death of fishes apart from unhealthy water, and we will be highlighting them.

Killing Beneficial Bacteria In The Aquarium

In aquariums, some bacteria used for breaking down waste lives in the filter and substrate. The bacteria can be unintentionally killed when changing water without adding de-chlorinator.


Aquarium maintenance

This is making sure your aquarium is working fine by consistently cleaning and changing the water weekly. When carrying out maintenance, make sure that the fish are carefully evacuated.



This is one of the major causes of fish fatality. A fish undergoing stress will be worn out, vulnerable to diseases, and will ultimately die.

Aquarium size

When stocking, it would be important to take the size of your aquarium into consideration. Overcrowding would lead to the suffocation of the fishes and trigger mass deaths.


Aquarium Setup

Stocking the wrong species together can lead to future fatality, as warm water-tolerant species fixed in cold water will be subjected to dire stress and eventually die. This is also true for cold-water species in warm water.


This reason may not seem like a problem, but it is. Fishes only need a well-sized meal to get them going. Overfeeding them can lead to severe complications, including pooping a lot and abandoning foods that eventually contaminate the water. This contamination causes toxicity which can be fatal to their survival.


Rough Travel

Moving your fish from its natural habitat when traveling can have negative consequences, and this is made worse if the traveling container does not have enough oxygen. The fish will be stressed and put in shock.

Old Age

This type of fatality is natural and cannot be helped. As your fish reaches towards the completion of its circle, death becomes inevitable.

Fish Died After Water Change

A frequent water change may be appropriate in the human sense, but this is not the case for fishes. Changing your aquarium water is essential for your fish’s health, but massive and frequent change can be detrimental to fish health.

Does water change kill fish? Not really. Does massive water change kill fish? The answer is yes. Fishes get used to a particular habitat. They build chemistry with it and understand it as their home. Over time, the waste from them, uneaten particles of food, dead plants from leaves etc., create changes in the water. They gradually get used to the water and take it as normal. Hence carrying out a massive change by draining this water means changing the complete makeup of the water and destroying their habitat.

Filling the drained aquarium with new water fills their senses with a new kind of surroundings which, more often than not, they find impossible to tolerate. This eventually kills them within a short period of time. Those that manage to survive will be subjected to dire stress and become vulnerable to diseases and infections over the next couple of weeks, with some dying off.

Hence, if water changes have not been carried out for a while, maybe in weeks or months, it would be ill-advised to carry out a total or massive water overhaul. However, for the long-term health of your fish, you should start a regular water-changing routine. You should first start small by changing just a little past five percent of the total water volume. Let the change settle. Repeat this process the following week and continue for several months.

With this pattern, you will be able to make your water fresh while keeping your fish safe. 

Healthy fish are happy fish.  Click this affiliate link to give your fish a healthy advantage.

How Long Does It Take For A Dead Fish To Float?

You may have wondered how long it takes the carcass of a dead fish to appear on a water surface. This is understandable as there is very little information on this.

When dead, fish carcasses normally sink for a couple of days before returning to the surface. However, there are some exceptions that we will take into consideration.

Some fishes, when they die, may float immediately within the space of an hour. This is caused by the bacteria responsible for the fish’s death. If the bacteria builds gasses inside the fish at the time of the attack, the fish after death will float immediately.

However, when they die, most fishes will sink to the bottom of the water because of their denseness. They will remain at the bottom until the process of decomposition takes place. During decomposition, gases (carbon dioxide) are produced in the fish body, accumulating until it becomes bloated. This forces the dead fish to the surface of the water.

The decomposition process varies with water temperature. It will take 3 days for the fish to float for warm water, while for cold water, it could take up to 8 days.

Click this affiliate link to order a special treat for your fish.

Not all fish will float to the water surface, and there are two reasons for this. First, a dead fish’s carcass, when at the bottom, will likely be attacked by scavengers, and such will never return to the surface.

Secondly, some fish just don’t float. There have been claims and several observations on dead fish carcasses decomposing and staying at the bottom where it would simply rot away. These cases, however, are extremely rare, with little information about the cause of this phenomenon. With more studies, though, detailed information on this occurrence will be provided.

Hazel Buckley
Hazel Buckley

About Hazel Buckley
Hazel is an animal enthusiast and educator who grew up on a farm which her parents owned in Ingogo, KwaZulu-Natal, South Africa.  The farm was situated right under the Majuba Mountains - the site where the Anglo-Boer War was fought. 

Disclaimer: Whilst every effort has been made to ensure that the information published on this website is accurate, the author and owners of this website take no responsibility  for any loss or damage suffered as a result of relience upon the information contained therein.  Furthermore the bulk of the information is derived from information in 2018 and use therefore is at your on risk. In addition you should consult professional advice if required.