Why Did My Fish Die? 11 Reasons

Imagine you’d always been fascinated by aquarist hobbyists and decided to join the team. You set up your aquarium, buy fish, and you start enjoying seeing them swim in your aquarium. Then suddenly, after a few days, you wake up only to find your fish dead. It can be such a heartbreaking experience. Perhaps you never prepared your aquarium well or may have made common mistakes that other beginner aquarists like you do.

The most common reasons why fish die include improper tank preparation, unsuitable aquarium size, stress, inappropriate tank mates, weak water conditions, poor tank management practices, improper feeding, diseases, human mistakes, chlorine in the water, and old age in fish.

Fortunately, this post will discuss 11 reasons your fish died alongside other helpful information. Read on!

Dead Fish Floating Upside Down

Signs of a Dead Fish

Here are the most common signs of dead fish in your aquarium:

  1. Dead fish won’t swim: Under careful observation, you may identify your fish floating in your aquarium or settled at its base. Both are signs of dead fish, although you may also notice sick fish floating on the tank.
  2. Physical appearance: Dead fish will have gray eyes immersed in its head. If you observe this sign, it implies that your fish probably died a long time ago.
  3. Your fish have gone missing: Missing fish can be an occasional sign of dead fish. Typically, if your fish tank lacks a lid, your fish may jump outside, and you may easily spot them over in the surroundings. 

These are signs you’re easily likely to observe in your daily visit to the fish tank. While my aquarium with various koi varieites can be a bit confusuing in terms of behavior, these signs cut across all fish types.

11 Reasons Why Fish Die

Here are 11 common reasons why fish die:

Causes of deathRemedies
Improper tank preparationCycle the tank before adding fish
Unsuitable aquarium sizeUse the right aquarium size for each fish 
Stress Eliminate the causes of stress
Inappropriate tank matesOnly pair up compatible tank mates
Weak water conditionsTrack and correct the water conditions
Poor tank managementAlways check and rectify the tank conditions
Improper feedingFeed each fish type correctly
Diseases Identify and treat diseases or call a vet
Human mistakesDouble-check everything 
Lack of dechlorinationTest and dechlorinate water periodically
Old ageEuthanize old and sick fish

1. Improper tank preparation

Improper tank preparation is a common reason for new tank syndrome that kills fish quickly. In the worst cases, your fish may die overnight or within a few days in a week. This mostly happens due to improper cycling of the tank.

A suitable fish tank should have vibrant bacteria colonies, which help in breaking down the waste in the tank to maintain the health of the water.

Also, these bacteria play a joint role with the fish by feeding and thriving on fish and their food, making the water conducive for the fish. Thus, this balance is essential in maintaining the health of the fish tank. 

In case you buy a new tank, these tanks lack the microbe colony. Thus, you should start cycling a new tank to establish the micro-bacteria colony. Typically, each tank comes with a cycling manual which you can use before adding fish. You should only add fish to the tank after it is fully cycled. Otherwise, your fish will die.

2. Unsuitable Aquarium Size

The fish often die since the aquarium size doesn’t match their space requirements. Anytime you purchase a fish tank, you should always consider the number of fish you want to keep inside and the available space for keeping it. 

After considering these two factors, you’ll be able to decide the appropriate size of the tank. You also need to consider additional features you may add to your tank when determining the suitable size.

Most beginner aquarists mistake acquiring an aquarium based on space and introducing fish without considering special needs. This eventually leads to complicated tank problems that make them lose their fish. Therefore, it would be best to consider such factors when hunting for a new aquarium.

3. Stress

Fish may also die due to stress, such as improper tank preparation, inappropriate size of the tank, and attack by tank mates. The majority of beginner hobbyists are unaware of possible signs of stressed fish. Luckily, here are the most common signs of stressed fish you should watch out for:

Extreme hiding: Like humans, fish hide when they are scared. If you notice fish trying to hide beneath the plants, décor, and the aquarium base, it is a common sign that they are under stress. This may happen due to aggressive tank mates, undesired tank conditions, and unprepared stocking options.

Illness: Stress leads to fish sickness. If you realize your fish are getting sick more often, then this is a sign that they are under stress.

Weight loss: Fish may get slimmer than usual due to stress. It can be challenging to weigh the fish regularly, but you can quickly notice when they are emaciated.

4. Inappropriate Tank Mates

As a beginner aquarist, you may choose the wrong tank mate for your fish. This can pose significant problems since fish have different behavior patterns. Some fish are territorial and will chase others, and others are aggressive such that they will enjoy messing with other tank mates. 

Also, other fish will form suitable mates with other species rather than their species. Therefore, it is essential to find a suitable tank mate for your fish through proper research to avoid this problem.

In some cases, the tank mates may include other types of aquatic life. For example, while kuhli loaches eat snails, it’ll be dangerous to put them in the same tank with asasin snails which may kill the fish instead.

5. Weak Water Conditions

A fish tank with unsuitable water conditions will lead to the death of fish. Weak water conditions are among the key reasons why fish die. Therefore, as a beginner aquarist, you should always aim at maintaining suitable water quality in your aquarium.

For instance, you should always keep track of your tank’s pH, nitrates, and nitrites levels according to your fish’s requirements. This will help to reduce unwanted problems.

6. Unsuitable Tank Management Practices

The aquarist hobby requires some little effort and maintenance. However, you need to ensure a sound way of handling things. If you run things otherwise, you will mess up your art. 

You only need to spend some time (a maximum of 50 minutes) maintaining a large aquarium. Smaller tanks with a size of 10-gallon will require little effort and time. 

You should always ensure the following monthly maintenance program:

Water changes: You should change the water in your aquarium to solve possible pollution. Changing dirty water with fresh water will remove all harmful and waste chemicals and enhance a fish-friendly bacteria colony.

Cleaning the algae: Even if your fish eat algae, you need to spare time each month to remove algae at the base of your aquarium. Algae may cover plants and the décor in your tank lowering its aesthetic value. Also, it may lead to the development of pest snails that may upset the balance condition in your aquarium.

Maintaining the filter: Water filters ensure clean water in your aquarium. Permanently remove gunk from your water filter at least once a month to ensure that fish-friendly and helpful bacteria thrive. 

Water testing: Testing the water quality in your aquarium monthly is a good practice to ensure that everything is functional. If you realize any unsuitable conditions, you can fix them immediately to prevent any possible problems.

7. Improper Feeding

Although you may enjoy watching your fish eating, it is advisable to feed them once daily. Ensure you provide the correct quantity of food they can eat within a meal. 

You should never overfeed your fish since overfeeding can lead to severe problems. For instance, excess release of waste and leftover food remaining uneaten to decay and alter water quality. 

Therefore, always ensure you supply the right food type and amount in your aquarium. Some fish such as cichlids eat strawberries and other fruits while other fish may not even touch such foods.

8. Diseases

Bacteria, parasites, protozoa, viruses, and other disease-causing microorganisms alter the health of fish. Sick fish will behave disturbed and you can identify such signs in different ways.

Fish mostly get sick due to dirty water and stress. Sick fish may show signs of moving for long durations and settling in one corner. The most common fish diseases include:

Dropsy: This is a common disease arising due to the build-up of fluids inside the tissues and body cavity of the fish.

Ich: A contagious parasitic infection transmitted from one fish to another.

Swim-bladder disease: Mostly occurs due to unsuitable water quality and stress.

Fin rot: Mainly occurs due to injury due to fighting and inappropriate water quality.

Flukes: These are parasitic worms that mainly alter the health of the fish’s gills. You can quickly identify them by a layer of mucus that covers the body or gills and the swift movements of the fish.

You can easily prevent these diseases by ensuring proper tank hygiene, avoiding stress and overfeeding, and maintaining suitable water parameters. Such simple actions will help keep your fish free from disease.

9. Human Mistakes

Beginners and experienced aquarists both make mistakes. The most common mistakes are overfeeding, feeding unsuitable food, and stocking errors. This means that fish may be deprived of the essential benefits they acquire from features such as light and filtration.

Such mistakes are easy to avoid by learning and improving your art. Thus, you can easily maneuver such errors in a matter of time.

10. Failure to Dechlorinate the Water

Direct water tap is unsafe for your fish unless it is treated. This water contains partial traces of chlorine which is toxic to fish. Chlorine can cause your fish to die within a few hours. 

Nonetheless, you can quickly treat water using a water conditioner to remove chlorine which may harm your fish.

11. Old Age

Old fish may also be a reason why your fish died. This may happen especially when your fish has a previous owner. Fish will die at a certain age irrespective of the care you give them. For instance, bettas are considered old at 4 years, while goldfish can live for up to 10 years.

At times, you’ll need to euthenize the fish or simply let them die by their own doing. It was sad losing my very first goldfish after years of care.


There are many reasons why you may find your fish dead. Fortunately, most of them are easy to manage and control. For instance, you can ensure the correct water quality in your aquarium by testing it to provide suitable conditions for your fish. 

Similarly, you can choose the right fish tank size to allow enough space for your fish. You can now watch out for those common causes of death for fish to emerge a successful fish hobbyist.


New Mexico State University. Understanding and Preventing Fish Kills in Your Pond.

Clemson University. Fish Kills and Aquatic Animal Health.

The USGS Water Science School. What causes fish kills?

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