Why Are My Fish Dying (12 Reasons Why)

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Fishkeeping is a fun and exciting hobby that comes with its own shortcomings. However, the main challenge many fish aquarists face while keeping a fish tank is aquarium tank fish deaths. 

There are a number of factors that contribute to aquarium fish deaths. In this article, I’m going to list the top 12 answers for why are my fish dying. I’ll show you how to prevent frequent aquarium fish deaths and how to overcome the loss. 

Keep reading!

Key Takeaways

  • Stress and disease are the primary reasons why fish will die
  • Poor tankmates and improper water parameters can lead to stress
  • Regular maintenance is a great way to keep stress down and keep your fish healthy

Top 12 Reasons Why Your Fish Are Dying

1. Improper Tank Setup

The most frequent cause of fish death is an improper aquarium. When the fish tank is not set up properly, most of the time, all or many of the fish perish. It primarily occurred with beginners. The fish may pass almost instantly in extremely severe tank conditions, but usually, it takes a few days or a week. It occurs as a result of improper tank cycling.

We tend to believe that aquariums are home to only fish, not realizing that a healthy aquarium also includes tiny organisms, of which a healthy bacterial colony is an integral part. Although they are distributed throughout the tank, the gravel and filter continue to hold the majority of them. They aid in the tank’s waste breakdown and maintain the water’s quality. 

Dirty Fish Tank

Bacteria have a symbiotic relationship with the fish by eating and surviving off the fish and their food. In return, the bacteria help to make the water in the tank suitable for the fish. Therefore, a balance between the two is necessary to maintain the tank’s health.

A microbial colony is absent from a new or fresh tank. As a result, you must cycle a new tank to establish the microbe colony1. A fish tank typically comes with an instruction book that explains how to cycle it.

However, I advise learning more about the nitrogen cycle and the various ways to cycle a new aquarium so that you are aware of the options and may choose the most appropriate approach. Avoid introducing the fish until the tank has completed its cycle.

Even if you’ve already cycled the tank properly, avoid adding too many fish at once in a fish tank. The germs require time to multiply, along with the quantity of fish in your tank.

Therefore, if you intend to add 15 to 20 fish to your aquarium, we advise adding four or five at a time over a period of weeks. This will allow the bacteria colony to grow at its own rate; hence, the death rate will be much less.

2. Stress and Disease

The most common reason for a fish’s death is stress and diseases. The stressed or diseased fish will most likely die if you don’t solve the problem. The main cause of stress in fish is the lack of tank preparation, which is imperative to maintaining an overall healthy fish tank. 

Surprisingly, all the ten reasons for your fish deaths cause stress in your fish in one way or another. However, here are some common signs that your fish is stressed out.

Surfing around the glass

If you see your fish continually swimming up and down and furiously moving around the sides of the tank, it may be because they are stressed. This is known as surfing around the glass

It demonstrates how the fish feels trapped inside the aquarium and longs to enjoy himself somewhere else. It might be due to the aquarium being too full, the water not being of good quality, certain species attacking it, or anything else.


Another significant factor contributing to stress in fish is illness.

Illness is also caused by stress. If a fish becomes ill frequently, it may be because it is experiencing ongoing stress. 

They are more susceptible to contracting various illnesses because stress negatively impacts immunity. Identify the origin of the stress since, if there is an ailment, one fish could spread the sickness to the entire school of fish, so be careful to know the cause.

Losing weight

If you find that your fish are getting thinner than they used to be, this indicates that they are losing weight for some reason. 

Although you can’t always weigh the fish, you can still detect if they are starting to lose weight. Even after eating a healthy meal, stressed fish may begin to lose weight and feel lethargic.

Excessive hiding 

When we’re terrified, we all tend to hide. Similarly, fish behave the same as humans by hiding when they feel uneasy or desire some calm. 

Your fish may be trying to hide under the plants, at the bottom of the tank, or beneath the aquarium accessories; if you spot them doing this, the causes are evident.

  1. Your fish are facing bully or aggression from other aggressive fish
  2. Small or inadequate tank size for your fish

Thus, before choosing any tank mate for your fish aquarium, be sure to research the behavior, the compatibility with other species, and the minimum tank size required to keep your fish healthy.

3. Aggressive Tankmates & Incompatible

Some fish species have a hostile nature. Other fish will battle with them. Tiger barbs are well-known for this type of behavior. They make the other fish in the tank anxious. That might be the cause of some of them passing away. 

Wolf Cichlid

To cater to this, I advise having a backup little tank handy when you set up your first fish aquarium. Take note of the fish community’s activities in the main tank. The combative ones can be moved to another tank.

4. Water Quality and Water Parameters

You can only imagine how crucial it is to maintain ideal water quality to keep fish disease-free and alive! Your aquarium’s water is the contained environment that supports your fish. 

Numerous variables, most of which are primarily related to unstable water characteristics, might result in a decline in water quality. 

If you haven’t been checking the water in the tank with aquarium test kits, a lot of guesswork is probably involved in determining why your fish keep dying

The majority of aquarium fish have a rather narrow range of water requirements that they must meet in order to survive. Therefore, crystal-clear water is not always a good indication of the quality of the water in a tank.

Here’s what you can do to maintain water quality and water parameters for a healthy fish tank.

  1. Every week, test the various water characteristics (pH, ammonia/nitrites, salinity, water hardness, Mg, Ca, etc.). 
  2. Keep pH levels stable and within an acceptable range for the fish species, you have in the tank rather than attempting to achieve the ideal pH level (or any other parameter). 
  3. If your fish keep dying, do this! To stop further pet fish deaths, act quickly when there are obvious warning signs (ammonia level spikes). This might entail performing a 50% tank water change right away, followed by careful monitoring of the ammonia/nitrite levels until the water quality is stabilized.

5. Wrong Aquarium Size

The right tank size for your setup depends more on the quantity and kind of fish you intend to keep in it than it does on where you intend to keep it. 

Fish can become so stressed out that they eventually die when kept in a tank that is too tiny to allow them to have free swimming spaces. Although a bit extreme, it is possible because stress increases the susceptibility of fish to illnesses and early demise. 

The most common reason for getting the wrong-sized tank is when a fishkeeper purchases pet fish when they are still young without doing any study on how big those fish can go or what their tank setup requirements are.

To avoid this, there are two things you should consider before buying the fish tank.

  1. The place where you’re going to place the tank
  2. Fish species you want in your aquarium.

6. Overfeeding In Enthusiasm

You might be tempted to feed your fish more and more as you learn how to keep a freshwater fish. 

The majority of fish species will consume everything that is presented to them. You could keep eating indefinitely if you are not stringent about how much food should be supplied at a time.

It’s possible that the fish won’t eat everything. The fish waste, along with waste, and uneaten food, will start accumulating towards the bottom of the tank. And if not removed, it taints or pollutes the tank water soon, which results in sick fish. 

Additionally, the fish will have a bloated tummy from overfeeding. They lack a bodily system that would allow them to detect fullness. They will therefore continue to eat, fall ill, and pass away.

Algae blooms are another issue related to overfeeding. A nitrogen-rich meal will make the tank greener. The tank will still appear unkempt and unattractive even if you have algae eaters. As a result of having more food, such as algae and uneaten fish flakes, snail populations may also increase.

Even though the bacterial colonies and microbes in the tank will work on the uneaten food particles, if circumstances are out of your control, the water in the tank will become toxic and harm the fish.

7. Poor Water Filtration And Equipment Failure

In an aquarium, dilution is the best defense against contamination. 

Simply said, that means you must discard some soiled water and replace it with clean water. By doing this, you improve the beneficial bacteria colony that is helpful to fish while also diluting garbage and dangerous toxins.

Make sure you don’t remove a lot of the old water in this phase because doing so will also get rid of the beneficial bacteria.

A monthly change of about 30% for aquariums with plants and 20% for aquariums without plants will produce good results. You can modify your water change amount and schedule based on your nutrient levels.

Even if you have algae eaters in your fish aquarium, you still need to get dirty once a month to completely remove all of the algae from your fish tank. 

Glass and decorative items are covered in algae and need to be cleaned frequently. Algae outbursts can cause a variety of issues, including deteriorating the tank’s aesthetics, poor water quality, supplying food for the growth of snails, and upsetting the equilibrium of your aquarium. 

Although you can find several chemicals and solutions to eliminate algae, you should avoid using them.

Use scrubbers and scrappers made specifically for fish tanks as an alternative. They contain a magnet, so you can clean them without putting your hands inside the tank.

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I love the Flipper. Just put it in your aquarium and clean from the outside. You can even move it to other sides of the tank with its flipper feature. Simply Amazing!

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How to keep your tank clean?

  1. Feed your fish only once or twice daily, depending on their nutritional requirement. Always feed them enough food that they can complete it in two minutes. Some fish actually do better fasting one day of the week to clear their digestive systems
  2. Clean the fish waste produced and food waste by occasionally siphoning the gravel in your fish tanks
  3. Regularly inspect and clean the aquarium’s filtration system. On the filter media, leftover food will accumulate and degrade the filter’s performance
  4. Adding fish that are bottom feeders is a good option for keeping a community tank clean

8. Not Keeping Up With Maintenance

This is a common reason why fish die. In most cases, ammonia and nitrites begin to accumulate in the aquarium water, resulting in ammonia poisoning. 

The introduction of too many fish into the aquarium at once may be the cause of the abrupt rise in the levels of harmful substances.

Large-scale fish introductions will quickly degrade the habitat, and many fish species won’t be able to adapt if you don’t set up a reliable maintenance cycle for cleaning the aquarium.

Carbon dioxide (CO2) and oxygen (O2) cooperate in fish aquariums. The amount of carbon dioxide in the tank’s water drops when oxygen levels rise and vice versa. This is a routine procedure. 

Due to the activities occurring in your fish tanks during the night, oxygen levels are lowest around sunrise.

The amount of oxygen increases throughout the day as the sun comes up, reaching its highest point at dusk. The carbon dioxide, which is at its lowest at this time of day, begins to climb after sunset and peaks soon before daybreak.

It’s crucial to prevent carbon dioxide peaks brought on by green algae blooms from causing your pond fish to perish. The most effective method for doing this is to utilize a UV clarifier.

How does CO2 enter your tank?

Aquatic life, including fish, snails, algae, and aquatic plants, create carbon dioxide through respiration (breathing). Additionally, it is created by decomposing organic debris on the pond floor, such as leaves, uneaten fish food, and algae blooms.

9. Changing The Aquarium Water Rapidly

The aquarium’s water has to be changed periodically. The full supply of water shouldn’t be replaced all at once, though. Partial water changes are generally best. 

Before adding the water to the tank, it should be treated. You may replenish the water in the tank by taking some out. Nitrites levels will rise, and your fish won’t be able to handle it if you’re completely changing the water or if you add fresh water without first treating it.

10 . Filling The Fish tank With Tap Water Directly 

Chlorine or chloramines are frequently present in the water you receive at home. The chlorine that has been dissolved in the water will be released after being set aside for a day, making the water cleaner. For the purpose of removing chloramines from the water, you must purchase specific chemicals. 

If you don’t take these safety measures, the fish will be harmed by the newly added water. I had the unfortunate experience of having a friend of mine kill her Betta Fish because she forgot to treat the tap water during a water change. Don’t make that error!

11. Old Age

The most common reason for the death of most fish is old age. Let’s be honest. Like us, fish are living creatures that will die someday in one way or another.

Especially if your fish is aged and has a history of ownership, it may have lived its life to the fullest, and hence your fish died.

Aging is inevitable regardless of how well you take care of your fish. You cannot escape becoming older. 

How long your fish will live totally relies on its breed. A healthy fish may live for ten years, whereas four years for bettas is considered ancient. Similarly, an unhealthy fish will not even survive a day. 

12. Rough Handling

Can you picture being outside when all of a sudden you are kidnapped and placed in a cramped space? 

Sounds exhausting, doesn’t it? 

If your fish was wild-caught, it traveled halfway across the world and now finds itself in a little tank at your neighborhood fish shop, which is a significant change. 

The fish you purchase can already be anxious from their lengthy travel. The final leg from the fish store to your house can also be deadly. And so, your fish die of stress and improper handling.


How Do I Stop My Fish From Dying?

Check the nitrate and ammonium levels in a new tank on a regular basis, and change the water as necessary to lower the levels to a safe level for fish. 

Rapid water changes: In a well-maintained tank with resident fish, plants, and bacteria, the water chemistry is carefully balanced.

Should I Change The Water After They Have Died?

Yes, you definitely should. No matter what caused your pet fish to die, you should always replace some of the water in the aquarium right after your fish died. Large volumes of ammonia will be released into the water by a rotting fish, and it is super harmful to other healthy fish in your fish tank.

Why Do They Die So Easily?

Fish can go into shock by sudden water changes and changes in water temperature, pH, or salinity, which quickly results in fish deaths. This is why it’s important to quarantine any new fish you bring home in a quarantine tank before putting them in your aquarium.

The temperature of the water in the bag and the water in your tank may vary significantly.

Why Are They Dying In My New Aquarium?

Because they are suffering from new tank syndrome. A new tank syndrome is a condition many fish keepers have to deal with when they newly set up the tank and don’t cycle it before adding fish into it.

Heavy quantities of nitrates and ammonium in the water can be lethal until a tank develops the proper water chemistry to sustain healthy fish. This condition is known as “new tank syndrome.” Fish may unexpectedly die until the natural microorganisms in the wYater are able to balance out these toxins.

Can You Save Them From Dying?

Yes, you can. With simple water changes and adjustments, the majority of dead fish may be quickly recovered. 

You must maintain water quality if you want to keep your fish alive and in good health. Most pet retailers sell water testing kits for fish tanks. You may use these tests to find any water issues, such as excessive ammonia levels or ammonia poisoning.

Why Are My Fishes Are Dying Suddenly?

There are a number of factors that contribute to the sudden fish deaths in your aquarium. However, a few known reasons are:

  1. Poor water quality
  2. Frequent water changes
  3. New tank syndrome
  4. No tank maintenance
  5. Poor water filtration and equipment failure

Final Thoughts

Like all pets, your fish has to go through the process of aging and eventually dying. However, sometimes, death comes too soon without any evident reason. And so, to help you figure out the reasons your fish die, I’ve written everything I know about fish deaths from my fish-keeping experience. 

Just make sure to provide your fish with pristine water quality and water parameters, and you’ll have no problem raising your adorable finned pets again.

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