Why do My Fish Keep Dying? 15 Reasons and Prevention Tips

As a fish owner, there is nothing more disheartening than watching your beloved pets die. Unfortunately, it happens more often than we’d like, and there can be several reasons behind it. Let’s take a closer look at the 15 most common causes of fish death and provide tips on how to prevent them, so your fish can live a long and healthy life.

fish keep dying

Aquarium in Not Cycled

One common reason for fish dying is when the aquarium is not cycled. Cycling is the process of establishing beneficial bacteria in the tank that break down toxins created by fish waste. Without this biological filtration, levels of ammonia, nitrite, and nitrate will rise, and your fish will struggle to breathe, experience stress and die.

To cycle your aquarium, you need to add a source of ammonia, such as fish food or pure ammonia, and monitor levels of ammonia, nitrite and nitrate over a period of several weeks. This cycle can take up to a month to complete, so patience is key.

Prevention Tips:

  • Test water regularly: Test ammonia, nitrite and nitrate levels in your aquarium using a reliable water test kit.
  • Add beneficial bacteria: Use products such as bacterial supplements or bio-seed to speed up the cycling process.
  • Gradual introduction of fish: Add fish gradually – start with a small number of hardy fish to allow the biological filtration to catch up.


Stress is another common reason why fish may die. Fish are very sensitive to changes in their environment and can quickly become stressed, leading to illness and death. Stress can occur due to various reasons such as overcrowding, changes in water temperature, pH levels or salinity, inadequate oxygen levels, or poor water quality.

Overcrowding can cause fish to become stressed since there is not enough space for them to swim and hide. Similarly, any sudden changes in water temperature, pH levels or salinity can result in fish getting stressed. Poor water quality, including high levels of ammonia and nitrite can also stress fish and make them susceptible to diseases.

Prevention Tips:

  • Maintain good water quality: Regularly check water quality and take measures to ensure that parameters are within safe ranges.
  • Avoid overcrowding: Make sure to maintain a reasonable stocking level to provide adequate space for all fish
  • Acclimate new fish: Gradually introduce new fish into the aquarium, allowing them to adjust to the environment.
  • Keep water temperature stable: Ensure that water temperature is maintained at a constant level.

Chlorinated Water

Chlorine is commonly used to sanitize tap water and remove harmful bacteria. However, this can be harmful to fish if not handled with care. Chlorine is toxic to fish and can damage their gills, making it difficult for them to breathe.

To prevent chlorine-related fish deaths, water used in the aquarium must be treated to remove the chlorine before introducing fish. This can be done in several ways, such as using tap water conditioner, which neutralizes the chlorine in the water.

Prevention Tips:

  • Use a tap water conditioner: Add a tap water conditioner while filling up the aquarium with new water. This will remove chlorine and other toxic metals that can cause harm to fish.
  • Let tap water sit: Allow tap water to sit for 24 hours before filling up the aquarium. This will give time for chlorine to evaporate from the water.
  • Use a water filtration system: Invest in a good quality water filtration system, which can remove chlorine and other toxic substances from tap water.

By following these simple steps, you can ensure that your fish are not exposed to harmful chlorine levels that can cause harm to them.

Small Tank

Fish require space to swim and hide in order to maintain their physical and mental health. Keeping fish in a small tank can cause stress and disease, leading to early death. A small aquarium can also cause water quality problems as fish produce waste that pollutes the confined water.

The general rule of thumb when it comes to tank size is to provide one gallon of water per inch of fish. This means that if you have a fish that grows to six inches, you will need a minimum of a six-gallon tank to keep him healthy.

Prevention Tips:

  • Get the right size tank: Purchase an appropriately sized tank for the number and size of fish you have or plan to keep.
  • Avoid overstocking: Do not overcrowd your fish tank as it can cause water quality issues and negative impacts on the fish’s health.
  • Provide adequate filtration: Use a filter that is appropriate for the size of your tank to help maintain a healthy environment for your fish.

By providing your fish with enough space, you can help to prevent fish deaths resulting from stress, disease and poor water quality.

Wrong Water Parameters

The wrong water parameter is another reason why fish can die. Finding the right pH and water hardness levels for your fish is important because it impacts their ability to eat, reproduce, and fight off illness. If the pH is too high or too low, it can cause fish to be stressed and sick.

Similarly, if the water is too hard or too soft, it can prevent fish from absorbing nutrients, resulting in stunted growth, weakened immune system, and increased susceptibility to diseases.

Prevention Tips:

  • Research your fish species: Study your fish species to gain knowledge of the ideal water parameters it requires to thrive.
  • Test water parameters regularly: Use a reliable water testing kit to test for pH, ammonia, nitrite, and nitrate levels, and monitor parameters to keep them within acceptable ranges.
  • Make gradual changes: Making sudden drastic changes to the water parameters can cause stress to the fish. Any changes to the water chemistry should be done gradually.

By keeping the water quality in check, you can minimize the risk of fish deaths resulting from the wrong water parameters.

Incompatible Tank Mates

Fish have different temperaments and needs, which means some species are more compatible with each other than others. Keeping incompatible tank mates can result in territorial fights, aggression, and stress – leading to injuries, illness, and even death.

Before adding new species to your aquarium, research extensively to find out which fish species are compatible with your current tank mates. Factors to consider include size, temperament, diet, and water requirements.

Prevention Tips:

  • Research before purchasing: Find out what species can coexist with your current stock before making any purchase.
  • Match sizes: Smaller fish can be bullied or eaten by larger species, so ensure that species added to the aquarium are of a similar size.
  • Variety in the aquarium: Having a diversity of species in the aquarium can reduce aggressive behavior, so consider adding different species that complement each other.

By ensuring that different fish species can coexist peacefully in your tank, you can avoid deaths resulting from territorial fights, stress or incompatible water parameters.


Overfeeding is a common mistake made by fish owners that can lead to poor water quality and fish deaths. When too much food is given, it can easily decompose in the aquarium and produce waste that contains harmful toxins, which increase ammonia and nitrite levels.

Overfeeding can also cause digestive problems to fish, leading to swim bladder issues, which affects their ability to swim and makes them prone to bacterial infections.

Prevention Tips:

  • Limit feeding time and portions: Give fish a small amount of food at a time and only what they can eat in 2-3 minutes. Feeding too much can lead to wasted food, which can pollute the water.
  • Avoid feeding too much live food: Live food such as worms will break apart and make a mess in the aquarium. Make sure live food is chopped and moved quickly to avoid excess feed.

Feeding your fish in small and frequent portions is a way to ensure they get the nutrients they need while reducing the risk of disease, health issues and death resulting from overfeeding.

Lack of Maintenance

A poorly maintained aquarium can lead to a variety of problems resulting in fish deaths. Without proper cleaning and water changes, waste and uneaten food can accumulate, leading to a toxic environment for fish. Over time, algae growth can block filters, and dirt can accumulate on aquarium surfaces, further reducing water quality.

Prevention Tips:

  • Regular water changes: Carry out regular water changes, removing at least 25% of the water in the tank once a week. This helps to maintain good water quality.
  • Clean the filters: Maintain the filter regularly, clean filter media or cartridges, and ensure the filter flow rate is within the manufacturer’s guidelines.
  • Clean the aquarium: Clean the inside of the aquarium with a sponge and a scraper to remove algae, dirt and grime that can build up on the glass or other surfaces.

By performing routine maintenance on your aquarium, you can maintain good water quality and a healthy environment for your fish. Regular cleaning and water changes can prevent fish deaths resulting from dirty water or clogged filters.

Lack of Water Changes

Lack of water changes can lead to a build-up of toxins in the water, releasing substances such as ammonia, nitrite, and nitrate. These toxins are harmful to fish as they can cause stress and illness, ultimately leading to death. Besides, the water can also become cloudy or form green algae, making it toxic and difficult for fish and plants to survive.

Prevention Tips:

  • Perform regular water changes: Change water at least once a week, removing at least 25% of the total volume of water.
  • Monitor water quality: Test water regularly to ensure that parameters such as ammonia, nitrite, and nitrate are within the recommended range.
  • Use aquarium plants: Aquatic plants can help absorb toxins and reduce nitrate levels through photosynthesis.

By performing regular water changes and water quality monitoring, you can maintain good water quality, prevent the build-up of toxins, and ensure a healthy environment for your fish.

Too Much Water Change

While performing regular water changes is essential, too much of it can also have adverse effects on fish health and lead to deaths. Drastic changes in water temperature, pH or chemistry can cause stress for fish and increase the risk of illness.

Performing very frequent water changes or changing too much water at once can also remove beneficial bacteria from the aquarium. Beneficial bacteria play a crucial role in breaking down toxins produced by fish waste, and removing them can disrupt the nitrogen cycle, making it difficult to maintain good water quality.

Prevention Tips:

  • Limit the amount of water changed: Change no more than 25% of the total volume of water at one refresh.
  • Make gradual changes: Any changes to the water chemistry should be done gradually to avoid stressing the fish.
  • Use a water conditioner: Use tap water conditioner when adding new water to remove chlorine and other harmful metals.

By keeping up with a regular water change routine and making only small changes at once, you ensure the aquarium environment remains stable, and your fish remain healthy.

Killed Beneficial Bacteria

The beneficial bacteria in an aquarium play an essential role in the nitrogen cycle as they convert ammonia to nitrite and nitrite to nitrate – the process referred to as cycling. Killing these bacteria can cause ammonia levels to rise, resulting in poor water quality that can kill your fish.

Some methods that can kill beneficial bacteria in the aquarium include using antibiotics, over-cleaning, and replacing filter media, and exposing the filter media to tap water that contains chlorine or other chemicals.

Prevention Tips:

  • Avoid over-cleaning and replace filter media only when necessary: Over-cleaning and removing or replacing filter media frequently will upset the balance of beneficial bacteria in your aquarium.
  • Avoid introducing harmful chemicals: Avoid exposing filter media to tap water that contains chlorine or other chemicals that can kill beneficial bacteria.
  • Avoid using antibiotics: Only use antibiotics when necessary, and avoid using them as a preventative measure.

By keeping these tips in mind and providing a stable environment for beneficial bacteria in the aquarium, you can prevent fish deaths resulting from killed beneficial bacteria.


Intoxication refers to the poisoning of your fish via toxic substances, either through accidental ingestion or absorption of the toxin through the gills. Some common sources of toxicity to fish include chemicals in tap water, soap, detergents, or exposure to pesticides.

Fish that are suffering from poisoning often exhibit signs such as unusual swimming behavior, lethargy, and loss of appetite. In severe cases, they may have open sores or bleeding.

Prevention Tips:

  • Use only safe and permissible aquarium products: Only use aquarium products that are specially designed for use with fish, and not to be harmful to them.
  • Keep harmful substances away from the aquarium: Keep cleaning products, pesticides, and other harmful substances stored away from the aquarium.
  • Check the water source: Ensure that tap water is free from contaminants, and treat tap water with a tap water conditioner to neutralize any potential harmful substances.

By taking precautions to avoid toxic substances and regularly monitoring your aquarium environment, you can prevent fish deaths from the ingestion or absorption of toxic substances.

Disease or Parasite

Disease or parasite infection may be another reason why your fish are dying. Fish discovered to have contracted disease or parasite infections can spread the disease to others in the aquarium. Inadequate water quality, poor nutrition, and stress can all make fish more susceptible to illness.

Some common diseases and parasites that can affect fish include Ich, Columnaris, parasites, and fungal infections.

Prevention Tips:

  • Avoid overstocking: Overcrowding can create poor health conditions resulting in stress and disease, so ensure your aquarium is appropriately stocked.
  • Regular monitoring: Regularly monitor your fish for signs of illness, such as cloudy eyes, rapid breathing, or a loss of appetite.
  • Quarantine new fish: Quarantine any new fish before introducing it to the community tank to prevent potential disease outbreaks.
  • Maintain clean water: Take measures such as regular water changes, proper filtration, and good food hygiene to maintain good water quality.

By ensuring that the aquarium environment is well-maintained and monitoring fish regularly, you can prevent fish deaths resulting from disease or parasite infections.

Wrong Handling of Your Fish

Fish are delicate creatures, and improper handling could lead to injury and stress, weakening their immune system, and increasing mortality rates.

Some common ways that fish are mishandled include netting the fish too roughly, lifting the fish out of the water by its tail, or using the wrong container without proper acclimatization.

Prevention Tips:

  • Use a soft net: Use a soft and fine-meshed net to avoid injuring the fish.
  • Use a container for transportation: Use a clean, water-filled container for short term transportation while ensuring that the water is suitable for the fish.
  • Acclimate new fish: Ensure that new fish are gradually introduced to the aquarium environment by slowly adjusting water conditions before transferring them to the tank.

By following these guidelines and handling your fish with care, you can avoid injuring and stressing the fish, resulting in early deaths.

Old Age

Fish, like all living things, have a limited lifespan, and old age may be the cause of death for some fish. The lifespan of fish varies from species to species, but most fish live between three and ten years.

The most common signs that a fish is reaching the end of its lifespan are sluggishness, loss of appetite, and a loss of color.

Prevention Tips:

  • Research the lifespan of your fish species: Various fish species have different lifespans, so research the lifespan of your species and provide appropriate care to help them reach their full potential lifespan.
  • Monitor your fish regularly: Keeping a check on your fish’s health and behavior regularly, and providing appropriate care according to age and lifespan can help increase a fish’s lifespan.

By providing good care and monitoring your fish regularly can extend their lifespan and make it more enjoyable for their owners.


Keeping fish alive can be quite challenging, but with the right knowledge and approach, you can drastically reduce the chances of their demise. Be mindful of the water quality, overfeeding, incompatible tank mates and ensure the aquarium is adequately maintained. What do you think are the major reasons for fish deaths? Share your thoughts in the comments below.

Updated: April 26, 2023

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