Why Fish Die in Aquarium – Know the Reasons

Dead Gold Fish In Small Aquarium to highlight the reasons for fish death in an aquarium

One of the most depressing experiences for you as an aquarium owner is when your fish die in the aquarium. You can’t digest how the playful Molly, whom you saw actively moving in the aquarium last night, is floating dead the next day.  A few hours later, an Angel fish dies, which continues to dishearten you more until you find the reasons.

Understanding the reasons behind these unfortunate events is important in preventing future losses. In this article, we will explore the most common reasons why fish die in an aquarium and how to avoid these issues. Stay tuned…

Is it common for fish to die in an Aquarium?

Unfortunately yes, it is common for fish to die in an aquarium, and is quite disheartening. Many factors contribute to this, including the stress of adapting to a new environment, poor water quality, improper care, and inadequate knowledge about fishkeeping, among others.

However, with the right information and practical measures, it is possible to create a thriving aquarium to significantly improve the lifespan and well-being of our aquarium fish and minimize fish deaths.

Dead Gold fish to highlight why Fish die in an aquarium

How do fish die naturally in an Aquarium?

Death is a natural and inevitable part of life, even in the aquatic world. Aquarium fish, like any other living creature, can die due to various reasons, like old age, genetic factors, disease, or weakened immune systems. However, the majority of aquarium fish deaths are preventable.

Aquarium fish, just like any other pets, require proper care and attention including good water quality, proper nutrition, and a stable aquatic environment.  However, in most cases, the death of aquarium fish can be attributed to preventable causes.

  A white illuminated Tip Bulb with yellow background to highlight a fact Poor water quality is one of the most common causes of fish deaths in aquariums. Poor water quality can lead to an imbalance of oxygen, carbon dioxide, and other essential nutrients, which can be fatal to fish.

Reasons Why Fish Die in Aquarium

There are several reasons why the fish die in an aquarium. Some reasons are biological such as aging, genetics, and weakened immune systems, for example. Besides, most fish die in an aquarium due to environmental reasons, including poor water quality, overfeeding, improper tank size, incompatible tank mates, disease outbreaks, stress, a lack of oxygen, and improper filtration. Let’s discuss them one by one to optimize our aquatic knowledge.

Poor Water Quality


Two types of water placed side by side to highlight the poor quality of water

One of the primary reasons for fish deaths in aquariums is poor water quality. Fish rely on clean and chemically balanced water to thrive. Ammonia and nitrite build-up, high levels of dissolved organics, and fluctuating pH levels can all have detrimental effects on fish health.

Ammonia, a toxic compound produced by fish waste and decaying organic matter, is one of the most harmful substances for fish. When ammonia levels rise in the water, it can cause severe damage to the gills, leading to respiratory distress and even death. Nitrite, another byproduct of organic waste breakdown, is also toxic to fish. High nitrate levels can interfere with the fish’s ability to transport oxygen, resulting in suffocation.

Regular testing and proper water treatment are vital to maintain optimal water quality. By monitoring ammonia, nitrite, and pH levels, aquarium owners can identify any imbalances and take the right actions to fix the situation. Water treatment products, such as dechlorinators and biological additives, can also help neutralize harmful substances and promote a healthy aquatic environment.

Improper Tank Size 

When it comes to setting up an aquarium, size matters. The size of the tank plays a significant role in the overall well-being of the fish. Unfortunately, many novice aquarists choose a tank that is too small for their fish and end up facing numerous challenges. Improper tank size limits swimming space, compromises water quality, and increases stress levels. Fish can die in aquariums due to all these factors.

Different fish have different needs when it comes to tank size. Some species, like goldfish, require large tanks due to their rapid growth and high waste production. Others, such as bettas, can thrive in smaller tanks as long as they are provided with sufficient activities and care. The best is to research the specific requirements of each fish species and provide them with ample room to grow and thrive.

Providing fish with plenty of room to grow and thrive is not only beneficial for their well-being but also enhances the overall aesthetic appeal of the aquarium. A well-planned and properly sized tank allows for the inclusion of natural decorations, plants, and hiding spots, creating a visually pleasing and harmonious environment.


Multiple Fish Diet to highlight that overfeeding can kill fish in aquarium

Overfeeding is a prevailing behavior among new aquarists. While it may seem like an act of kindness to offer excessive food, it can have terrible consequences for the fish. Overfeeding leads to uneaten food decay, increasing ammonia and nitrite levels, and causing water contamination. Developing a feeding schedule based on the needs of the fish and regularly removing any uneaten food is important to prevent overfeeding.

Incompatible Tank Mates

Different fishes including sharks to highlight the Incompatible Tank Mates

Introducing incompatible tank mates is the most ignored reason that can increase the chances of fish deaths in an aquarium. This also can result in stress, aggression, and even physical injuries among the fish.

Remember, each fish species has unique compatibility requirements based on their size, temperament, and habitat preferences. Researching and carefully selecting compatible tank mates will promote a harmonious environment and reduce the risk of fish deaths in the aquarium.


Virus around the fish pot to highlight disease in aquarium fish

Disease outbreaks can quickly upset an aquarium population if not detected and treated quickly. Bacterial, viral, and parasitic infections can spread rapidly in confined spaces, compromising the health of fish. Quarantining new fish, regular observation, and implementing appropriate preventive measures, such as maintaining water quality and providing a well-balanced diet, can significantly reduce the risk of disease outbreaks.


Big Gold Fish Shouting at a small fish to highlight stress in aquarium fish

Stress often goes unnoticed but can have severe results on fish health. Factors such as sudden environmental changes, overcrowding, aggressive tank mates, and poor water conditions can all contribute to stress. What you all need to do is to minimize stress by creating a stable and calm environment that promotes the fish’s well-being. This will help reduce their vulnerability to stress and prevent sudden deaths in an aquarium.

Fish Can Die in an Aquarium Due to Lack of Oxygen

Water pump to highlight oxygen level in aquarium

Fish are dependent on oxygen for their survival. They extract oxygen from water through their gills, which is why it’s important to ensure that the water in their aquarium is properly oxygenated. Inadequate oxygen levels can lead to respiratory stress and even death in fish.

To keep the aquarium well-oxygenated, proper ventilation is essential. Keep the water surface agitated to promote oxygen exchange between the water and the air. You can do this by using air pumps, powerheads, or filters that generate water movement. These devices help break the water’s surface tension and increase the surface area, allowing for more oxygen to dissolve in the water.

In addition, regular maintenance of the equipment is also decisive to ensure that the oxygenation system is working correctly. Air pumps and filters should be cleaned and checked for faults regularly to avoid poor oxygen levels in your aquarium.

Temperature Fluctuations 

Thermometer in an aquarium to highlight temperature fluctuation- a potential reason for fish death in aquariums

Never compromise on temperature stability for the well-being of fish in an aquarium. Rapid changes in temperature can cause stress and compromise the immune system, making fish more susceptible to disease. Select the right heaters, regularly monitor and maintain the water temperature within the recommended range for the fish species, and avoid exposing the aquarium to direct sunlight or drafts. These measures can maintain optimal water conditions which will ensure that your fish don’t die in the aquarium.

Let’s Wrap it Up!

Knowing the reason(s) why fish die in aquariums is the first step to suppressing the death ratio and maintaining a healthy aquatic environment for fish. The delicate balance of a well-built aquatic habitat can be disrupted by several reasons including poor water quality, overfeeding, and improper tank size.  The social dynamics within the tank matter—mixing incompatible tank mates can lead to stress and fatalities in the aquarium. In addition, disease outbreaks, often a consequence of poor hygiene or stress, pose a significant threat.

To ensure that our aquatic friends thrive in an aquarium, maintaining optimal water conditions by regular monitoring of water parameters that include temperature, filtration, and good pH level along with a balanced feed, can further protect against common dangers.

Dr. Musya Edward
DVM | Consultant Surgeon (pets and large animals)

Dr. Musya Edward is a highly skilled and assertive veterinary surgeon who is committed to the well-being of pets and large animals. With his extensive knowledge and experience in veterinary medicine, he ensures that every pet receives the best possible care. His expertise in surgical procedures is unmatched, and he leaves no stone unturned in ensuring the complete recovery of his patients. His passion for animals extends beyond the operating room and you can find him actively engaging with the pet-loving community through his insightful articles and expert advice on pet care.