Reasons for Freshwater Fish die in a Tank and Their Preventions

There could be one or more reasons that cause fish to die in a tank. A few basic reasons like incorrect tank setup, diseases, stress, and overfeeding can cause this. As a beginner, it might be hard for you to understand why fish are dying. But there should be something that you did or didn't do. I am listing down each and every condition here so if you learn and follow the process to fix the problem then this problem will be solved.

Let's get started...

  • Heavy concentrations of nitrates and ammonia in the water can be fatal in a tank before it has developed the appropriate chemistry to support healthy fish. Natural bacteria in the water will eventually balance out these contaminants, but fish may die unexpectedly until that balance is reached. Ammonia Poisoning can kill the fish.
    • To avoid this, test a new tank for nitrate and ammonium levels regularly, and change the water as needed to reduce levels to safe levels for fish.
  • Rapid Water Changes: The water chemistry in a healthy, established tank is carefully balanced with resident fish, plants, and bacteria. Rapidly changing large amounts of water will disrupt the chemistry and shock the fish, resulting in death. Water must be changed regularly, but large, abrupt changes are harmful.
    • To avoid this, slowly change the water, swapping only small amounts at a time and waiting 2-3 days before changing more water so the fish can acclimate to the gradual chemistry changes. 
  • Water Quality: The overall water quality of a tank is an important factor in fish health. The salinity of the water (for saltwater aquariums), pH levels, filtration efficiency, and other quality issues must all be kept within ideal ranges, or any of them could be the cause of dead fish. 
    • To Prevent: Research the appropriate water quality for your fish and take steps to maintain appropriate quality levels to protect the health of your fish. 
  • Temperature Changes: Although most fish can tolerate a wide range of temperatures in the tank, sudden or dramatic temperature changes can cause stress, making the fish more susceptible to illness. Extreme changes can be lethal in a hurry. So Setting up a perfect temperature is highly important.
    • To Avoid: Check the tank heater for proper operation daily, and keep it away from anything that could affect its temperatures, such as heating or cooling vents, sunny windows, or draughty areas.
  • Unexpected Toxins: Even small amounts of toxic contaminants, such as bug spray, hand lotion, perfume, soap, cleaning chemicals, and other seemingly innocuous materials, can be fatal to fish. Fish will suffer if the water becomes contaminated with these items.
    • To Prevent: Use a good cover and keep toxins away from the tank to protect it from unintentional contamination. Before entering the water for any reason, always wash your hands with unscented, non-antibacterial soap.
  • Overfeeding: Fish do not require three meals per day and overfeeding not only wastes food, but spoiled food can contaminate the tank and upset the delicate chemistry. Fish only require a few mouthfuls of food per day, and while they may beg, they do not require additional treats. Check out How much should I feed my Fish and at What time?
    • To avoid this, establish a strict feeding schedule and stick to it. Adjust the feeding amounts until the fish consume all of the food in 1-2 minutes, and then stop feeding them.
  • Unhealthy Fish: If fish were already stressed, sick, or in poor condition before being introduced to a tank, even the best aquarium conditions may not be enough to keep them alive.
    • To Avoid: Be aware of the symptoms of common diseases in the fish you intend to buy, and always buy fish from experienced, reputable breeders or dealers. Choose only the healthiest fish in the best condition so that they can withstand the stresses of transportation and reintroduction into a new tank.

Why are my freshwater fishes dying?

 Fish can be wonderful pets, with their vibrant colors and soothing swimming motions, but it can be unsettling and upsetting when fish die for no apparent reason. However, there is always a reason, and understanding the most common causes of aquarium fish deaths can assist fish owners in preventing fatal mishaps in their tanks. There are a variety of reasons why the freshwater fish in your tank may continue to die. Overcrowding, diseases/infections, stress, insufficient tank cleaning, and other factors are among them. Most fish keepers have a rocky start, especially if little research is done before doing all the fun stuff (setting up a tank, Aquascaping, etc.). 

Each fish species is unique in its way. Most aquarists become enamored with the vibrant bursts of color and grace that populate our fish tanks. 

That is why having several fish die in a row can be very upsetting, especially after putting in a lot of effort to get an aquarium up and running with all the necessary equipment. Fortunately, most fish deaths are the result of avoidable circumstances. There is nothing you can do to prevent a fish from dying of old age, but mass deaths are always caused for concern.

12 most common causes of your fish's death:

  1. New Tank Syndrome
  2. Choosing The Wrong Tank Size
  3. Unsuccessful Transfers
  4. Overcrowding
  5. Excessive Water Changes
  6. Water Quality & Unstable Water Parameters
  7. Sudden Changes In Water Temperature
  8. Overfeeding
  9. Not Cleaning The Tank Enough
  10. Deep-Cleaning A Tank
  11. Diseases, Fungal Infections, Brooklynella, And Bacterial Infections
  12. Stress in Fish

What to do when your fish are dying?

Ascertain that your aquarium's environment is suitable for the fish.

Many times, people forget what kind of fish they have and keep a standard set up for all of their fish. This is a huge mistake because some fish will not be able to survive in the environment you’ve created for them due to a genetic need for a different environment.

Examine the water in your aquarium.

If you put a fish in cold water when it prefers warm water, it will die. It will also die if you keep saltwater fish in a freshwater tank.

Check to see what kind of water stability your fish prefers. A fish that prefers still water, such as a betta, may die from the stress of being in a tank with too many filter bubbles. Fish like African cichlids prefer alkaline water, so putting them in gentle, acidic water like the cardinal tetras will kill them.

Examine the contents of your aquarium

If you have a pleco, it will need a lot of crevices and hiding spots in the tank to feel safe. If your tank is empty, it will become stressed due to a lack of places to swim into.

It's always better to be safe than sorry, so do your homework on your fish’s water and tank preferences before purchasing it.

If you are unable to provide your fish with the tank conditions that it requires, you will stress it out to the point where it will either suffer for weeks before dying or will have a shortened lifespan due to constant discomfort.

Set up the aquarium properly.

It takes a lot more than just filling a tank with water and a de-chlorinator to get fish to thrive in your tank.
There's a lot more to setting up the right conditions for your aquarium than that, which includes adding the right chemicals, installing a filter, and making sure it's the right capacity for the number and type of fish you have.

You must first ensure that your tank contains the proper types of bacteria that a fish requires to stay healthy. If you put your fish in a newly filled tank, it may be dying as a result.

If your fish is dying due to a lack of bacteria in its environment, your only option is to try to place it in a tank that has already been cycled. When aquariums are cycled, they have already been exposed to the bacteria that your fish requires to stay calm in its environment.

What causes fish to die suddenly?

Having a fish tank is enjoyable and rewarding, but seeing the fish die is extremely frustrating.

Consider getting a beautiful tank, filling it with water and all of the necessary and expensive equipment, and then introducing your prized fish. When the fish finally start swimming and dancing in the tank, it's a mesmerizing and awe-inspiring moment. But the next morning, one of your prized fish has become entangled in the filter intake and has died. What comes next? 

When we have healthy fish, some of them will die unexpectedly. However, because we know the fish were healthy and eating properly, determining the cause of their death becomes difficult. It is critical to understand why they died to prevent it from happening again. On this page, I've listed 22 different reasons why fish can die.

Reasons why your aquarium fish died unexpectedly:

  1. The aquarium had not been cycled. 
  2. The fish were not properly acclimatized.
  3. Ammonia burst
  4. Oxygen deficiency Aggressive tankmates
  5. Rapid changes in water parameters
  6. Diseases and parasites
  7. Contaminants in the water supply
  8. The tank is too small.
  9. Newly purchased fish were already sick.
  10. The new fish had not been quarantined.
  11. Lighting with a high intensity
  12. Feeding fish
  13. Not given a varied diet
  14. Slamming into the aquarium substrate
  15. Failure of equipment
  16. Too much cleaning of the aquarium
  17. The aquarium is not cleaned frequently enough
  18. The water has not been dechlorinated.
  19. A visual barrier is required.
  20. Overfeeding
  21. Pockets of ammonia in the substrate
  22. Medication Overdose

Fish, like humans, can contract diseases. They become infected with parasites, bacteria, viruses, protozoa, and other disease-causing microorganisms, which cause a variety of problems in their bodies. However, if your fish is behaving strangely, there could be another reason.

How to save dying fish?

If you believe your fish is exhibiting some unusual behaviors or traits and is in danger of dying, you should examine it closely to determine what the problem is and whether there is a solution. Is there anything you can do to help your fish survive?

Examine the Body

Begin by scrutinizing the fins, tail, and abdomen. Take note of the body color, whether it is fading or developing spots, and examine the eyes for any bulging or abnormalities.

Take Notice of How It Moves

Is your fish moving or still? How and in which direction is the fish swimming? If you notice something out of the ordinary in your fish's behavior, you may only need to change the aquarium environment to put it at ease.

When fish are stressed, they frequently change their behavior. Observation is the key to comprehension.

Here are some of the main factors that can cause saving your fish.


Keep in mind that your fish’s aquarium is their world and home. If something in the aquarium is bothering your fish, it means it will have to deal with it all of the time, with no breaks.

The Water

Learn about the specific water requirements of the fish you’re raising.

Aquarium Decoration

Some aquarium fish like to hide, so they need a lot of places to swim if they want to feel safe. Your fish will be stressed if your aquarium has very little decoration that meets this need.

Chemicals, Filters, and More

It is critical to set up your aquarium correctly. You’ll need a filter that can handle the number of fish you intend to raise, as well as the proper aquarium chemicals.

Aquarium Dimensions

Do you have too many fish for the size of your tank? Overcrowding can result in an excess of waste in the water, and the good bacteria that break down waste become overburdened. As a result, your water will become toxic, and your fish will perish.

Friends and Aquarium Companions

Placing fish that should not be kept together is a common blunder. Fish, like people, have a variety of personalities. Some fish are laid-back or timid, while others are social butterflies, and still, others are aggressive and territorial.

What to do when a fish dies in your aquarium?

A dead fish should be removed from its tank as soon as you learn of the incident. This is because when a fish dies, it immediately begins to decompose, which can contaminate the water in the aquarium. The contaminated water could then kill the other fish in the tank. 

Ammonia will be released into the water by a decomposing fish.

Ammonia levels that are too high are hazardous to the other fish in the tank and may poison them, rendering the aquarium uninhabitable. When a fish dies in an aquarium, the other fish in the tank will most likely eat it after a while.

If the fish died as a result of a disease, the corpse is dangerous to the animals that eat it.

There's another reason why you should never leave a dead fish in the aquarium for days:

When a fish dies in your aquarium, here are five things you should do. This is done to keep other fish from dying.

  1. Remove the fish's corpse from the tank.
  2. Examine the aquarium water for elevated Ammonia levels.
  3. Replace some of the contaminated water.
  4. The tank with a beneficial bacteria starter.
  5. Look into the cause of death.

Why did my fish die after a water change?

The fish are not alone in the tank; there are plants and some good bacteria as well, and all three interact to form an ecosystem. The toxic ammonia in fish waste is broken down by bacteria into simpler, less toxic components, which are then taken up by plants. 

Large or infrequent water changes in an aquarium may kill the fish, but that doesn’t mean you shouldn’t strive to keep the tank’s water quality healthy. Aquarium water changes are beneficial to the tank’s aquatic inhabitants, but they must be done correctly. If your aquarium fish die after a water change, it is most likely due to the sudden change in water parameters: 

Fish can adapt to changes in their environment over time. Changing a large portion of aquarium water, on the other hand, may significantly alter the parameters of the remaining water inside the tank. The fish may be unable to adjust to the abrupt change in these parameters and may become severely stressed or even die as a result. This could occur even with a minor water change if a significant amount of time has passed since the last time you did it. 

During a water change, a variety of things can go wrong. 

Here are some of the water parameters that could be to blame for your dying fish:

  1. The nitrate levels in the water were too high before you changed it. 
  2. The new water's temperature was too low for the aquarium.
  3. You either forgot or added a Dechlorinator too late.
  4. You vacuumed the entire substrate and/or switched out the filter media.
  5. Your tap water contained an excessive amount of saturated gas.

Should you kill a dying fish?

If your fish has been suffering from a severe illness and no treatment has been effective, euthanasia may be the best option. It may appear cruel to end your fish’s life, but it may be the most compassionate thing you can do – especially if the fish is stressed and in pain. A dying fish is greatly comforted by having clean, warm water as well as a safe and quiet environment free of bright lights and loud noises. To avoid stomach pain or discomfort, a dying fish should be separated from any other aggressive fish in their tank and not overfed. 

Water changes will not affect the growth of the beneficial bacteria in your bio-filter. In this case, removing your sick fish will not help. Taking sick fish out of a well-established, cycled tank is frequently a good idea.

Is it bad luck when your fish dies?

Dead fish have been found to represent, among other things, death, self-sabotage, lack of progress, missed opportunities, and avoidance, depending on whether they are seen in your waking life or symbolic dreams. The natural death of fish in a tank is said to be the end of your problems. The dying fishes, according to aquarium Vaastu, take away the problem with them. You should, however, replace the fish as soon as possible.

You should not be concerned if a fish dies; simply replace it with another, as it is believed that the death of each fish reduces your problems. It is strongly recommended that the fish feed be planned and fed by a single person. The tank or bowl should be cleaned regularly.