What do My Fish Keep Dying? How to Prevent it?

For beginner fish owners, there is nothing more disheartening than fish kills. Even experienced aquarists are frustrated by seeing a lifeless fish floating on the water’s surface.

If your aquatic pets have been dying successively, it may be time to identify the causes and address them. Fish will not display expressions, and the death of your pet may catch you off-guard.

What do My Fish Keep Dying?
What do My Fish Keep Dying?

The common reasons for fish kills include:


Fish, like people, are prone to stress, mostly derived from their environment. Stressed fish are, in turn, vulnerable to diseases and other severe conditions. The bodies of most fish can heal themselves, and parts like tails and fins regrow after trauma. This ability is, however, reduced in stressful surroundings.

The causes of stress in a fish tank are varied. They include sudden substrate changes, bullying from other fish, inappropriate water parameters, inadequate physical space, insufficient oxygen, and poor nutrition.

Deviation from normal behavior is the most reliable tell-tale sign of stress in fish.

Some species will hide all the time, while others will feed less-aggressively than before or refuse food altogether. If you notice your fish gasping for breath near the surface, the tank may be low on oxygen levels.

You can also check for cuts and wounds on your pet since they would typically heal quickly unless the animal is stressed.

If your pet gets sick, they may be stressed because a healthy immune system would prevent them from contracting an illness in the first place.

Chlorinated Water

Aquarists are often discouraged from using tap water, which is often the cheapest kind of water to use. Most regions add chemicals like chlorine in their water supply to kill bacteria.

Low chlorine concentrations damage the gill structure of fish species. As the level rises, the fish will be observed swimming erratically or trying to leave the water.

The constant pain from the damage caused by chlorine will ultimately hurt your fish. Concentrated levels of the element will rapidly kill your aquatic pets and leave you wondering about the cause of death.

If you have to use tap water, leave in a bucket for a day to allow the chlorine to escape it into the atmosphere. This method is ineffective in regions where water has chloramine.

Chloramine has been growing in use because it is more stable than chlorine. It is made of ammonia and chlorine, and it cannot be removed through boiling water or letting it sit in a bucket. Chlorine and ammonia are both toxic to fish, making chloramine a contemporary headache for pond owners.

Small Aquarium

Aspiring fish owners are commonly mistaken to think that small aquariums are easy to maintain. The practice of keeping species like bettas in bowls and mini tanks will harm your pet.

The primary concern with small aquariums is poor water quality. Fish expel waste, some more than others, depending on the kind of species you have kept. The waste releases ammonia, which is then converted to nitrites and nitrates.

The waste will quickly get concentrated unless frequent water changes are made. Few aquarists can keep up with the kind of maintenance that small quarters need. The problem is exacerbated by adding more fish or overfeeding pets. Bad water quality is known to kill fish quickly.

Small aquariums are also unstable environments. The water conditions are constantly changing and stressing the residents. It is harder to maintain stable water conditions with a small water volume, and most small aquariums will lack filtration.

Aquarists who invest in small tanks will typically fill them close to the surface, which leaves a little surface area for gas exchange. Fish need well-aerated environments, which is not often the case with small aquariums.

There is always a risk of territorial hostility among fish species in a cramped space. Some fish species like having a portion of a tank’s territory to themselves, and they will be hostile to their tankmates.

Bullied fish will, on the other hand, develop high levels of stress and contract diseases. Keeping fish in big tanks promotes their health, and they will be better equipped to ward off infections.

Ammonia Poisoning

Ammonia poisoning is among the most common killers of aquarium fish.

The ideal ammonia levels in any setup with aquatic pets should be zero. In the wild, the levels of ammonia are diluted by the abundant water volume. In an aquarium, the fish and the waste the release exist in one space, which is why filtration systems are so advanced. Filtration should be accompanied by frequent water changes to keep ammonia levels non-existent.

Ammonia poisoning typically occurs when the PH levels in a tank rise and interrupt the nitrogen cycle. Decomposing organic matter in the tank will also contribute to the presence of ammonia.

The matter can be in the form of uneaten food, fish waste, and plants. These products attract bacteria that also release ammonia and compounds on the problem.

In some areas, tap water will contain chloramine instead of chlorine. The former consists of ammonia and chlorine, both of which are deadly to aquatic life.

Fish species that are experiencing ammonia poisoning will be observed gasping for air at the surface. The gills will appear swollen and red while the fins develop red streaks.

The fish may be lethargic or will swim frantically to indicate distress. The animal’s tissue will also be affected, leading to hemorrhage and death.

If you detect ammonia poisoning early, it will be helpful to lower the water’s PH and do a 50% water change. A couple of water changes may be needed to get rid of ammonia completely. There are chemical products that neutralize ammonia levels. Most aquarists will restrict feeding to control the spike in ammonia.

When starting, ensure your tank is well-established to cultivate the bacteria that break down into ammonia into nitrates.

Incompatible Fish

Housing incompatible fish species in a single tank is a ticking time bomb. You should research on the level of aggressiveness in the fish you want to keep to reduce hostility in your tank. There are chances, however, that things could go wrong even in a carefully-planned setup.

Aggressive fish have been known to kill their tankmates and frustrate their owners as a result. Hostility in captivity often results from territories. Some species view boundaries as a place they can retreat to, hide, or mate, and they can claim a corner, rock, or cave.

Most of these species are especially territorial to members of the same species or gender. They will rely on shape, color, or pattern to decide if a particular fish is of similar species.

Fish aggression is often not apparent. Aggression is mostly observed on the bodies of the bullied species. Look for nipped fins, missing scales, scratches, and scrapes. The bullied fish can also spend more time hiding and away from view. In worse cases, the vulnerable pet will jump out to its death.

Most aquarists will remove the aggressive fish if they notice bullying behavior.


Overfeeding your aquatic pets can lead to a plethora of problems. Most aquarists use commercially-available dry foods to feed their pets. These products will expand in the animal’s stomach, and if they are over-fed, they may die. Species like the African cichlid can develop a fatty liver, which is deadly.

Overfeeding commonly pollutes aquarium water. Leftover food will subsequently decompose and release ammonia and reduce water quality. This process utilizes oxygen, and it will cause the water to be less-aerated. Acids, which lower the PH, are also released.

The large amounts of dissolved materials and elements like nitrates encourage an algal bloom. Aquarists typically observe an accumulation of red and blue-green algae that impede on the water quality. Cloudy water is another scenario that accompanies the overfeeding of fish.

In extreme cases, overfeeding will result in clogged filters as the filtration system tries to clean the high concentrations of waste.

Inadequate Aquarium Maintenance

Experienced aquarists know the importance of regular water changes even in an aquarium with a powerful filter. Fish kept in a tank swim in their waste and leftover food, products that release harmful elements like ammonia.

Ensure you do weekly water changes from the moment you get the aquarium to avoid problems from the very start. Replace 10-20% of the old water with a fresh supply. Aquarium maintenance also includes cleaning the substrates, removing dead plants, and washing the glass.

Disease or Parasite

Around 90% of the diseases that affect fish are linked to a weakened immune system. Common causes of this include sudden water changes, inappropriate diet, and poor water quality.

Numerous internal and external parasites affect aquarium fish. The most common one is Ich, which manifests as white spots on the animal’s body and fins. The parasite is extremely contagious, and new fish species can introduce it in the tank. Other parasitic diseases include Oodinium, Chilodonella, Flukes, and Nematode Worms.

Aquarium fish are also vulnerable to bacterial and fungal diseases as well as viruses. You can detect some diseases on the bodies of your pets, which is why you should monitor them regularly.

Observe if the fins or tails have rot, and check for ulcers, wounds, and other abnormalities on their bodies. A diseased fish will also exhibit unusual behavior like rubbing on rocks and other objects, twitching and swimming frantically.

Fish diseases can be hard to control, and your pets can be dead before you even know it. Most conditions have medicines and treatments that can be used, although you should be careful to avoid over-medicating the creatures.

Old Age

Different fish species have different life spans, which can substantially vary when compared to the animals in the wild. Bettas, for example, reach four years. If your pet had a previous owner, they would naturally live for a shorter time.

What to Do When Your Fish is Dying?

Most aquarists have been faced with a suffering fish and the choice of alleviating their pain.

The practice of euthanasia is quite controversial in human circles, and some fish owners will let nature into their hands and not intervene in any way. Other aquarists, however, believe that the task of alleviating their pet’s suffering lies in their control.

If you decide to kill your fish, the next step will be to select a method of choice. These methods are often divided into what is considered humane and what is not.

How to Euthanize a Fish?

Euthanasia methods vary with the kind of setup you have, including marine and freshwater tanks. The most common types are:

Anesthetic Overdose

An anesthetic overdose is one of the most acceptable means of euthanizing dying fish. The popular products used are Aqua-Sed and Eugenol.

Aqua-Sed should be administered at a higher dosage to kill your pet quickly. Leave the fish for some time to ensure that they are dead. Larger fish will typically take more time to succumb to the overdose. Some fish owners will decapitate the animal once it is deeply anesthetized.

Other fish owners used acidic products like MS 222, which will cause the more painful acidosis.

Clove Oil

The sedative clove oil is popularly used to euthanize aquarium fish. It is available in chemists and is recommended by the American Veterinary Medical Association as a human method to kill fish.

You can easily make a clove oil bath at home for your pet. Get a small container and add several drops of the product depending on the size of the fish. The common dose used is 400mg/l to water. The oil will not easily mix with water, and you will have to shake the container until the contents become a milky white color.

If the dosage is lethal enough, the fish will lose consciousness and stop breathing. If they are still swimming around, add a stronger does. The process may take a while, but it is often preferred for its humanness.

Physical Intervention

The use of blunt trauma is another quick way to kill fish.

To perform this method successfully and humanely, the first blow needs to be decisive. The idea is to disable the pet’s nervous system quickly so that they do not feel anything.

If you are wary of gore, place your fish in a piece of aluminum foil and fold it over. It can be disheartening to see guts and blood flying across the room. When you hit your pet, aim for the head, and if done right, they will immediately lose consciousness. If the blow is strong, and the fish is small, the first step will kill them.

If the fish is still alive, aquarists will quickly follow it up with pithing, where its brain is pierced to kill it instantly. This method can be tricky to use if you miss the brain. You can pierce horizontally above the eyes, or seek information beforehand. Refrain from decapitating the animal without pithing because the head of the fish may remain alive and aware for a few seconds.

Chances are that your fish will wriggle around if it is active, making the use of blunt trauma hard to perform. Most aquarists will use anesthesia before using force.

Inhumane Methods

Flushing is considered an inhumane way of killing fish, and it should be avoided.

A fish will encounter sudden changes in water conditions down the toilet, and will immediately get stressed. If it lives past this initial shock, it will be surrounded by human waste down the sewage drains. Flushing a fish is only subjecting it to a slow death as it drowns in people’s slurry.

Another unacceptable method is the injection of carbon dioxide into the aquarium water. The fish will subsequently suffocate and spend their last minutes gasping for oxygen. Novelty aquarists have also been known to leave their pets on dry land and cause them to suffocate to death.

While ice baths are sometimes used on smaller fish, the method may be inhumane for larger fish. Ice crystals will typically cause pain when they touch the gills. Some species like goldfish are also cold-tolerant and will survive for a while before it gets uncomfortable.

Your fish will also suffer greatly if you throw them in boiling water or attempt spine breaks.

Verify Fish Death

After implementing your chosen method of euthanasia, you should ensure the fish is actually dead. A fish is generally regarded as dead ten minutes after the last indication of gill movement.

How to Dispose of Dead Fish?

Flushing the remains of fish is not a respectful way to dispose of your beloved pet.

Some aquarists choose to bury their fish, although you should use a decomposable bag.

You can also scout for crematories that will cremate your pet’s remains. Your local vet can direct you to a pet crematorium, or you can look for veterinary clinics that handle such services.

If you decide to throw the fish in the trash, use two plastic bags that are sealed to contain the bad smell.


If your fish keep dying, it is time to identify and deal with the causes. They can include inadequate maintenance, insufficient space, presence of ammonia and other harmful compounds, and incompatible fish.

Experienced aquarists commonly tout stress as the biggest contributor to fish kills. Stress makes the immune system of a fish weak and renders them vulnerable to diseases and attacks. If your fish is beyond help, there are several humane methods you can use to put them down.

Updated: November 23, 2019

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