Why Do My Fish Keep Dying? (15 Common Causes + Solutions)

Wondering why your fish keep dying? This guide is for you!

Any keeper will be disheartened when their fish keep dying in the tank, especially if they can’t immediately put their finger on the cause of the deaths.

Most fish deaths are caused by human error. It could be that the keeper or someone else in charge makes an oversight that results in fish deaths. Alternatively, it could be that the keeper is a newbie with little information and experience in keeping the fish safe and healthy.

Whatever the case, you need to be proactive to stop the deaths of your fish. Unfortunately, panicking or worrying will not provide you with the necessary solutions.

Instead, you need to equip yourself with information on what’s causing the deaths. This puts you in an excellent position to put an end to these unfortunate occurrences.

This post gets you started on the causes of your fish dying in the tank, and what you can do about it.

dead fish

Why Do My Fish Keep Dying? 15 Common Causes and Solutions

#1 – New Tank Syndrome

When you set up a new tank, you need to give it time to allow the good bacteria to build up and settle before putting in the fish.

A good number of inexperienced keepers lose their fish because they put them in the new tank too fast, before the tank has gotten cycled.

Usually, a tank without good bacteria experiences a swift buildup of ammonia. Good bacteria break down and neutralize toxic waste, protecting the fish from harm.

If your fish has died suddenly after introducing him to the new tank, it could have died of ammonia poisoning.

This is a common phenomenon, referred to as new tank syndrome.

The Solution?

Don’t add fish into a new tank if you’ve already lost some in this enclosure. Instead, allow the tank some time to get properly cycled.

Also, test the water for toxins such as ammonia, nitrates, and nitrites. In case you find any anomalies, change the water and add a bacteria starter into the tank.

Once the bacteria get to work, the tank parameters will easily stabilize, and your fish will be safe.

#2 – Using the Wrong Tank Size

You should choose a tank size depending on the number of fish you intend to keep. If the tank is too small, you put your fish at a very real risk of facing premature death.

Different fish have various tank size requirements. While some fish, such as the betta, will be fine in a 5-gallon tank, this tank would be stressful to a larger species.

Using the wrong tank size is one of the leading causes of stress in fish. A stressed fish faces a myriad of health challenges, such as infections and disease.

A stressed fish does not feed properly, leading to a compromised immune system.

Also, housing your fish in too small a tank increases pollution levels. The water quality is compromised, increasing the chances of your fish dying from toxins.

If you’re unsure about the size of tank you need, go with bigger one.

The Solution?

Find out the appropriate tank size for the type of fish you keep. If some of your gentle giants are dying, it could be that their accommodation is inadequate.

It would be best to move them to a larger tank where they’ll be more active and stress-free.

Alternatively, you can shift some fish and leave the others in the original tank. This should make life comfortable for both sets of fish, and they will be less stressed.

#3 – Fluctuations in Water Temperature

Most fish are highly sensitive to temperature changes. If the water temperature falls outside your specific fish’s range, it leads to stress, disease, and death.

Water temperature can fluctuate in two ways, either upwards or downwards. Upward fluctuations are caused by positioning the tank wrongly.

For example, it could be in direct sunlight or near a heater. It could also be that the tank lights were left on for too long.

Downward temperature fluctuations are caused by a draft, keeping the tank next to a cooling vent, or a faulty heater.

If you have noticed any of these issues in your fish aquarium, then you know why your fish are dying.

The Solution?

Position the tank appropriately, so it is not in direct line with the sun. Also, install a time to monitor and control the lights on/off functionality.

Keep your fish tank away from any heating or cooling elements. Also, ensure it is not affected by a draft coming into the room.

Control the water heater with a digital thermometer to ensure that the temperatures stay within the acceptable range for the tank size and fish type.

#4 – Species Incompatibility

Say that everything in the tank is as it should be. The water parameters are okay, and the tank is toxins-free because you clean it regularly.

If your fish are still dying mysteriously, you need to look at this issue from another angle. Are the tank mates in the enclosure compatible?

Some of your fish may be dying because of ongoing fights or bullying. Finally, the victim becomes so stressed that he falls ill and dies.

You could have also brought together two fiercely territorial species. But, unfortunately, this is not good for either of them because they keep fighting to their death, or the more powerful one may kill the weaker one.

As such, you must understand your fish’s various personalities before bringing them together.

Introducing an aggressive, violet species in a meek and docile fish community would be disastrous.

The Solution?

Conducting due diligence before putting the fish together will save the day here. Research the personality traits of the fish you intend to house together to determine their compatibility.

If they are not compatible, think of housing them differently. A good number of fish species can coexist peacefully.

But, it takes good research work to find out which ones these are. So, ask questions from your breeder and read quality articles online.

Armed with the correct information, you can make the right decision and save your pet fish from premature death.

#5 – Poor Transfer Practice

If the fish die shortly after introducing them to the tank, rule out the issue of new tank syndrome. Also, you need to ensure that the transfer is done successfully.

Fish are supposed to be relocated in a certain way, failure to which a host of things can go wrong. For example, putting your fish in an inappropriate bag for the transfer can have dire consequences.

It would be best to hold the fish in an appropriate container, which supports your pet’s pH and temperature requirements.

Although the fish will not stay in this container for long, it needs to be comfortable during the ride.

At the same time, you need to acclimate your fish correctly to water conditions once you reach your destination.

Special care should be taken when transferring small or fancy fish, as they are the most vulnerable.

The Solution?

Transfer the fish in a suitable container, ensuring all tank parameters are correctly observed. Once at home, acclimate the fish to the new environment before putting them into the tank.

This will prevent new fish from suffering from shock and dying prematurely.

#6 – Absence of Beneficial Bacteria

As earlier noted, good bacteria help create healthy conditions in the tank for the fish’s survival.

If beneficial bacteria are lacking, it means waste products will build up very fast, leading to an increase in toxins.

All fish suffer greatly when exposed to high levels of ammonia, nitrates, and nitrites. This is where good bacteria come to the rescue.

Beneficial bacteria break down and neutralize the waste products before they produce poisons. In the interest of your pet fish, you keep a healthy amount of these bacteria in the tank.

The Solution?

When good bacteria die, your fish soon follows suit. As such, encourage the growth of good bacteria by regularly adding a de-chlorinating substance to the water.

This substance will eradicate chlorine, a major killer of beneficial bacteria.

#7 – Poor Water Quality

The tank is your fish’s permanent home, and the water is its dominant survival mode.

Anything that affects the water’s quality will directly impact this animal’s health and general wellbeing.

As such, it is your responsibility as a pet parent to ensure that the water quality is of the highest standard.

This should not be hard, considering you want nothing but the very best for your pet.

Water quality in a fish tank can be messed up by so many things. The only way to stay on top of things is to ensure regular testing with the appropriate aquarium test kits.

Remember, the water doesn’t have to be crystal-clear to be of the best quality. You need to test to verify that everything is okay.

The Solution?

Use the right aquarium testing kit to regularly test for all the water parameters. For example, is the pH range acceptable for the type of fish you keep?

What about salinity, water hardness, ammonia, nitrates, Ca, and Mg? If you check on these weekly, you’ll safeguard your pet from suffering unnecessarily or dying early.

#8 – Excessive Water Changes

You should change about 30 to 40% of the water in the tank weekly. However, if the water is excessively contaminated or unclean, you can go up to 50%.

A water change of 70% weekly is considered overkill. If anything, repeatedly doing this will send the aquarium into a new tank syndrome, inadvertently compromising your pet’s health.

A new tank syndrome means the tank lacks enough good bacteria. As a result, the buildup of toxins and poisons will be rapid, and it will be a matter of time before your fish succumbs.

The Solution?

If your fish has done as a result of changing the water too often, go back to the beginning. Treat that tank as if it was a new one and start by encouraging the growth of beneficial bacteria.

Once it has stabilized, you can put in new fish – but make sure you test the water daily.

Moving forward, do only water changes of about 15% as you add nitrifying bacteria until all conditions are right.

Schedule water changes every 2 weeks (about 25%) until you’re sure the bacteria levels are back to normal.

In the meantime, don’t put in any new fish before the beneficial bacteria culture is acceptable.

#9 – Diseases and Infections

Have you considered that your fish could be dying because of bacterial, fungal, or viral infections? Disease-causing parasites are known to threaten and take away the lives of pet fish.

These parasites may come with new fish arrivals that are carriers. Alternatively, the parasites are introduced into the tank when you introduce something new – such as a toy or live aquatic plants.

If the new décor you bring into the tank is not properly sanitized and washed, it may be the source of these parasites.

Indeed, parasites and diseases can access the tank from any source – including aquascaping tools and fishnets.

The Solution?

Before putting them in the community tank, new fish should be quarantined for a couple of weeks. This gives you an excellent opportunity to monitor their health, and take intervention measures if necessary.

At the same time, try to thoroughly interrogate the health and wellbeing of any new fish you intend to purchase.

This way, you won’t have to spend money on fish that could introduce diseases to your other stock.

All fixtures, décor, toys, fishnets, and aquascaping tools should be sanitized before using them in the tank.

Put any live plants in an observation tank before transplanting them to the aquarium. A week or two is enough to tell you whether they are healthy.

10 – Overcrowding

Do you keep adding more and more fish into the community tank? Unless the tank is big enough, you could be in for trouble!

Overcrowding happens when you put too many fish together. The good bacteria will be unable to catch up with the continued increase of waste products and the resultant toxins.

When too many fish are in the same space, the issue of ammonia poisoning becomes very real. Also, overcrowded fish don’t have enough spots to hide and be alone.

The result? Increased stress levels, disease, and death.

Unfortunately, most first-time keepers imagine overcrowding cannot happen with small species of fish.

But, the truth is that it affects both large and small.

The Solution?

If you want to keep many fish in a community tank, ensure the tank is large enough to accommodate them.

Also, allow for the buildup of nitrifying bacteria before adding new fish into the tank. This gives the beneficial bacteria the potency to deal with the influx of new waste from the fresh fish.

Add enough live plants and decorations into the tank to give your fish adequate hiding spots. This is necessary as it helps these animals handle stress.

#11 – Overfeeding

Some fish are ravenous eaters and will keep eating as long as the food is available. Unfortunately, this leads to bloating and constipation, common health conditions that kill fish prematurely.

Also, overfeeding creates an unhealthy imbalance in the tank because of all the leftovers that sink to the bottom of the tank.

Uneaten food soon starts decomposing and wreaks havoc on the entire tank ecosystem. This is one of the most significant contributors to nitrates, nitrites, and ammonia buildup.

Overfeeding your fish means the culture of nitrifying bacteria will be overwhelmed; they won’t break the waste in good time to save your pets.

The Solution?

Fish should be given as much food as they can consume within two minutes. Any leftovers should be removed from the tank within 5 minutes after the feeding session.

Fry should be fed about 3 to 4 times a day. Adults and juveniles should be fed once or twice a day. Anything more than this should be considered overfeeding.

Regularly clean the tank’s filtering system to remove dirt and grime. This system has to work efficiently to prevent the buildup of unwanted food materials.

Adding bottom feeders to the tank ensures no food stays hidden in the gravel and other crannies.

#12 – Deep Cleaning the Tank

If your tank has been dirty for too long, don’t compensate by over-cleaning. The best you can do is gradually clean it until you bring it to acceptable levels.

Over-cleaning the tank will likely cause fish deaths faster than a dirty tank. However, this is not to say that you should allow your fish tank to grow hopelessly dirty.

Deep cleaning the tank gets rid of the culture of nitrifying bacteria your pet fish need to survive. The fish will die without the beneficial bacteria, no matter how sparkling clean the tank is.

Nitrifying bacteria are a critical biological filtration system that your fish tank cannot do without. So, instead of doing the changes to the tank all at once, do them gradually.

For example, do a 25% water change today and change the filter media next week. Then, wash the gravel the following week and rinse the decorations the week after.

This way, you won’t upset the vital balance your fish needs to survive.

The Solution?

If your fish died because of over-cleaning the tank, treat that tank as new. This means you need to cycle it to encourage the growth of healthy bacteria.

At the same time, schedule tank maintenance so that you don’t have to do everything at once.

#13 – Improper Tank Cleaning

If you don’t clean the fish tank well enough, it will cause problems. Small tanks are particularly susceptible to a buildup of ammonia and other toxins every time you skip the cleaning schedule.

Dangerous algae and nitrates quickly increase to harmful levels when you miss the cleaning by as less as two times.

A well-cleaned tank has nitrate and ammonia levels of about 0 ppm. Anything above this is considered dangerous.

The nitrate levels should be at 10 ppm or lower. If your fish tank does not reflect these readings, it means you don’t clean it well enough.

The Solution?

Cleaning the tank is a duty that cannot be put off. You need to devise a proper cleaning schedule and follow it religiously.

This ensures that you neither over-clean nor under-clean the tank

#14 – Elevated Stress

This is the most significant cause of death in fish. Surprisingly, your fish does not die of stress, but the health conditions stress causes.

A stressed fish is open to all kinds of opportunistic infections. In addition, its immunity is severely compromised, making it impossible for this animal to ward off infections and diseases.

Stress in fish can arise from any of the issues we’ve discussed above. Other causes include improper feeding and environmental disturbances.

The Solution?

Ensure that your pet fish is well-adjusted and balanced. This means taking care of every aspect of his life.

As a pet parent, don’t take anything about your pet for granted. Does he seem lethargic, spending most of his time at the bottom of the tank? Find out why?

Is he refusing to eat? Then, perhaps, a change of diet is called for.

Add enough aquatic plants and decorations to give nervous fish proper hiding spots. This is one great way of helping your fish handle stress.

#15 – Old Age

Have you considered that your pet fish could have died of old age? Unfortunately, like humans (and all animals), age will eventually catch up with your pet fish.

Likely, your fish has led a full life, and it’s now time to depart this world. This should sadden you because, after all, all living things have to pass on.

Some fish live for 3 years, others for 7, and some for 12 years. The old age of your fish is determined by its species.

The Solution?

The best you can do in this situation is to spend time with your dying pet. Although watching your pet’s last moments may be difficult, you need to treat this animal with care and compassion.

Sit by the tank and talk to him, if that’s what you have been doing when he was healthy and hearty.


Discovering that your fish have been inexplicably dying in the tank can be saddening. This is more so if you had grand plans for your relationship with this pet for the remainder of its natural life.

But, instead of spending lots of time mourning the loss, be proactive and find out what’s happening. You’ll realize you can do something to prevent further deaths.

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