10 Most Common Reasons Why the Fish Die

10 Most Common Reasons Why the Fish Die

Having a fish tank is fun and rewarding, but, at the same time, seeing the fish dying is deeply frustrating.

Imagine you get a beautiful tank, put water and all the essential and expensive equipment inside it, and introduce your precious fish too. It’s a mesmerizing and awestruck moment when the fish finally start swimming and dancing in the tank. But the next morning, you find one of your precious fish stuck to the filter intake and become lifeless. What next?

We understand how heartbreaking this moment could be, especially when you did your 100% for the upkeep of your fish, and it seems they die without any reason. You don’t understand what went wrong, and you just want to know the cause of their death at any rate. In such situations, feeling discouraged is normal, but you should take it as a lesson that still there are a few things that you skipped learning.

Every aquarist should take this as a chance to re-examine and relearn the tank management along with tank and fish care practices. It is true for both- beginners and experienced fish keepers.

Here, we are going to put light on some common reasons why the fish die so early. It may help you diagnose and rectify some of the major issues that are leading to the fish death.

10 most common reasons why the fish die


Improper tank is the most common reason for fish death. Most of the time, when the fish tank is not prepared properly, all or a lot of the fish die. It majorly happened with the beginners. In really bad tank conditions, the fish may die overnight, but generally, it happens over a few days or a week. It happens because the tank is not cycled properly.

We think that only fish live in aquariums, lesser knowing the fact that a healthy tank also has microscopic organisms, among which a vibrant bacteria colony plays a crucial role. Although they are spread all over the tank, they mostly remain concentrated in the gravel and filter. They help in breaking down the waste in the tank and keep the water healthy.

Bacteria play a symbiotic relationship with the fish by feeding and living on fish and their food, and in turn, the bacteria make the water in the tank suitable for the fish. Therefore, a balance between the two is essential to keep the tank healthy.

A new tank is devoid of a microbe colony. Therefore, when you get a new tank, you need to cycle it to start to build the microbe colony. Generally, a fish tank comes with an instruction manual that provides information on how to cycle it. But we suggest researching a bit more about the nitrogen cycle and the different methods of cycling a new aquarium so that you know the various ways and the way best suited for you. Only once the tank is cycled should you add the fish.

Another thing to avoid here is adding too many fish at a time, even if the tank is properly cycled. It is because the microbes need time to grow with the number of fish in your tank. So, if you are planning to add 15 to 20 fish to your aquarium, we suggest adding four or five a week over several weeks and gives time for the bacteria colony to develop at its own pace.


Another reason for the fish’s death is stress. Stress may happen to the fish due to multiple reasons, among which reason number 1 (lack of tank preparation) is prominent. Combating stress and managing it is an important way of keeping the fish healthy.

Out of the 10 reasons we are going to discuss, almost all of them give stress to the fish in one way or another. Beginners are often not aware of the symptoms that fish show due to stress. Some characteristic signs include the following-

Surfing around the glass– If you notice your fish is repeatedly swimming up and down and frantically moving along the sides of the tank, the chances could be due to stress. It shows that the fish wants to enjoy at someplace else other than the tank, but feeling stuck inside. It could be because the aquarium is overcrowded, the quality of water is not good, there are certain species attacking it, or anything else.

Excessive hiding– We all hide when we feel scared. The same way fish too hide when it feel uncomfortable or want some peace. If you notice that your fish is trying to hide under the plants, at the base of the tank, or below the aquarium décor items, it could be because of the stress. A few reasons behind hiding could be-

  • Another fish is bullying the other fish in the tank
  • Territorial issues that are poor or unprepared stocking choices
  • Undersized tank

Certain fish like loaches, catfish, and plecos show hiding behavior more. So, before you select any fish for your tank, make sure you learn its behavioral pattern and its compatibility with other fish.

Weight loss– If you notice your fish getting slim than before, it means they are shedding weight due to some reason. You cannot take the weight of the fish every time, but you can easily tell if they are becoming emaciated. If the fish is under stress, they may start losing weight even after are taking good diet.

Illness– It is another prominent reason for the fish to get stress and vice-versa. Stress leads to illness. If the fish is getting sick more than often, it could mean that it is under constant stress. Since stress negatively affects immunity, they can easily get different types of diseases. Make sure you know the reason for the stress because if it is an illness, one fish may transmit the disease to the entire stock.


Many times the fish die because the size of the tank is not as per their space requirements. So when you go to buy the tank, there are two things that you need to know in advance-

  • Space you have to keep the fish tank
  • Fish you want to have in your tank

When you know the area of space you have to keep the tank, it will help you decide the right size of the tank. Also, you must know which species of fish you want and the essential equipment needed for their survival. It will again help you gain knowledge about the size of the tank.

Some beginners often make the mistake of getting the aquarium on the basis of the space and then introducing any fish with no knowledge about their special needs. This ultimately leads to big tank problems and you may lose some beautiful fish.

You may have read or heard somewhere the “one-inch-of-fish-per-gallon” rule. It means that you can keep one adult fish for one gallon of water in the aquarium. For instance, if you purchased a 20-gallon tank, you can keep 10 fish of size two-inch.

This formula works well if the fish is of small size and does not grow bigger. However, if the fish grows bigger, this formula becomes completely gibberish. Would you like to keep a pair of two feet long fish species in a 55-gallon tank? Obviously not, as it doesn’t make any sense and does not solve the purpose of keeping an aquarium.

Some fish really get so big with the time that they are unmanageable in a smaller tank. Therefore, until you have not researched well about the fish and how much size they would attain after their full growth, it is of no use to buy any species. Some fish really come with an expensive price tag, and you don’t want to lose them, right?

So, whether you are a beginner and buying a new tank or just want to add a new species of fish, either way, it is important to understand the needs of the fish and the size they attain once matured. A small tank can cause huge stress that will reduce immunity and promote illness. Also, the tank will get polluted easily, leading to premature death.

  1. Incompatible tank mates

It is not necessary that all tank mates will gel well. Selecting the wrong take mates may create big problems for the beginners. Every species of fish show a different behavioral pattern. While some territorial fish will run after every other fish in their territorial area, some fish are violent in nature and love to mess around with the other tank mates. At the same time, some fish are fine with other mates but may probably have problem with their own kind of species.

Therefore, it is important to know the behavioral pattern of the fish you are going to purchase, to make sure that they are a fit for your tank. An experienced aquarist could guide you the best when it comes to fish’s behavioral patterns. Take enough time to understand it to escape the situation of losing your precious water animals. You may ask the pet store owners, experienced hobbyists and do enough homework by reading multiple articles on the fish and gain knowledge. Purchase a fish only when you are double sure that it is compatible with other fish species.

  1. Poor water conditions

Whenever the tank has bad water or water not suitable for the fish’s health, it leads to their death. Poor water condition is a big reason why most of the fish die in a fish tank. Therefore, if you are an aquarist, the priority should always be to maintain the water quality. This, in turn, won’t put fish under stress and never affect their immunity or health. It can avoid the premature death of the fish.

The best thing you can do to check the water quality is to buy a testing kit in which you can take readings of most of the critical parameters of the water. You can know the level of ammonia, nitrates, nitrite, and pH and adjust if anything is going out of the range. If you have a freshwater fish tank, keep the level of your ammonia and nitrite at zero and the nitrates level below 20 ppm for the best results.

pH is an important parameter that can give or take life to a fish. There are many chemicals available in the market with which you can alter the tank’s pH. While a majority of tropical fish can cope up to the different levels of pH in a properly maintained tank, try to keep it within the range so that your new fish can easily adjust in a new environment. A little swing up and down is tolerable, but a balanced pH will always ensure the health of the fish.

Make sure you take regular readings of the tank’s water parameters and note them down somewhere in your diary. If something goes wrong in the tank, you don’t have to guess as the water-quality readings will reflect everything.

  1. Improper feeding

Feeding in the right proportion and at the right time is very important. It is advised to feed your fish only one time in a day. Make sure feeding only with the quantity that they can eat within a few minutes. Make sure you always give quality food flakes as they are made up of high-quality ingredients. While scavengers love to feed on sinking pellets, plecos and otos like to munch on algae wafers in their feeding schedule.

If you are giving special food to the fish, offer them in moderation. Some aquarists even include a day of fasting once a week for the fish on which they are not fed anything.

One thing that you have to keep in mind is to never overfeed your fish. Make sure to provide them fresh, well-nourished, and healthy diet and the fish will remain happy forever. Give them a variety of food like veggies, and thawed frozen foods that include freeze-dried foods and blood worms. Research and learn before you feed anything for the best results.

Many beginners make the mistake of overfeeding the fish. It can lead to a number of problems. Firstly, your fish will produce more waste if you feed more to them. Also, a lot of food will remain uneaten that decays and alter the water quality. Although the bacterial colonies and the microbes in the tank will work on the uneaten food particles, if things are not under your control, the tank’s water will get toxic that will cause health problems in fish.

Another problem associated with overfeeding is algae outbreaks. A food rich in nitrogen will turn the tank greener. Even if you have algae eaters, the tank will look dirty and unaesthetic. You may also witness a sudden rise in the number of snails that grow after suddenly having extra food like algae and uneaten fish flakes.

  1. Inappropriate tank management practices

The aquarium is one of those hobbies that require very little maintenance and effort. However, you need to do things the right way. Once things are done properly and the tank starts running correctly, you don’t have to worry about anything. All you need is to spend around thirty to fifty minutes for the maintenance of a big tank. A small tank of size 10-gallon may require a little more time and effort.

The monthly maintenance schedule should include-

  1. Water changes- When it comes to an aquarium, the best solution to pollution is dilution. It simply means that you have to remove some old and dirty water and replace it with clean and fresh water. In this way, you are diluting the waste and harmful chemicals and improving the fish-friendly bacteria colony. In this step, make sure you do not remove a lot of old water as it will remove the beneficial bacteria as well. If you have planted aquariums, around 30% monthly change and 20% for the unplanted ones will give good results.
  2. Vacuuming the gravel– Gravel lies at the bottom of the tank. All the uneaten food, fish waste, debris and gunk get entangled in the gravel. Sometimes the content of this material is so large that even the scavenger fish can’t handle all of it. It is when a little vacuuming of the gravel will help cleaning it. But make sure that you don’t get rid of each piece because it is a good dwelling place for fish-friendly bacteria. You can buy a quality gravel vac that will suck the extra gunk lying on the gravel. Vacuum the gravel while changing the water.
  3. Cleaning the algae– Even if you have fish that are algae eaters, you have to roll up your sleeves once a month to remove the entire algae from your fish tank. Algae cover the glass and décor items that require regular cleaning. Algae outbursts can invite a lot of problems like degrading the aesthetic of the tank, providing food for the pest snails’ development, and upsetting the balance of your aquarium. You can find different chemicals and solutions to remove algae but prefer not to use them. Instead, you can use scrubbers and scrappers designed especially for the fish tanks. They are equipped with a magnet, so you don’t have to insert your hands in the tank for cleaning purpose.
  4. Filter maintenance– Filter is an important part of the aquarium. It filters the water and provides clean water to your tank. Make sure you remove the gunk out of the filter. Do it at least once a month. However, at the same time, ensure that beneficial or fish-friendly bacteria still live there. Do not change or replace the parts of the filter until not prescribed by the company.
  5. Water testing– Testing the water once a month or periodically is a good practice to ensure that everything is working fine in your tank. If you notice anything going south, you can work on it immediately to curb the upcoming problems.

If you are performing these tasks once every month, they go a long way in battling all the issues that can affect the life of fish.

  1. Diseases

Just like human beings, fish, too, get diseases. They get infected with parasites, bacteria, viruses, protozoa, and other disease-causing microorganisms that cause a number of ailments in their body. However, if you find your fish acting weird, there could be another reason behind it as well.

Fish are tiny creatures with small and primitive brains. They can do a number of silly things that are unexpected. For instance, betta fish often act like dead by not moving its body for long durations. Likewise, there are fish that love to enjoy a lot of time in just one corner of the tank. It doesn’t mean that they are unwell or ill. So, if your fish is acting strange and weird, do not panic.

But we cannot neglect the fact that fish too get diseases. The two biggest reasons for fish illness are stress and dirty water. While some healthy fish can fight off the infection and get healthy, the weak ones may die. Therefore, it is important to watch for the signs of stress in your fish.

Some common diseases of fish include-

  • Ich– It is a parasitic infection that passes in one fish from another fish.
  • Dropsy– When the fluid builds up inside the body cavity or tissues of a fish, it causes the disease called dropsy.
  • Fin rot– It may occur due to some injury that happened while fighting off with aggressive fish or due to unclean and inappropriate water quality.
  • Swim-bladder disease– Swim bladder disease occurs when the water quality is not correct and the fish is under huge stress.
  • Flukes– Flukes are a class of parasitic worms that mostly affects the gills of the fish. You will notice a layer of mucus covering gills or body and rapid movements of the fish.

You can easily avoid most of the fish diseases by keeping the tank and water clean, avoiding overfeeding and stress, and maintaining the proper water parameters. These alone will help immensely in keeping the fish diseases free.

  1. Human Errors

To err is human. Whether you are a beginner or a pro, you could make mistakes, and the repercussions may vary. In the worst case, the fish will die. Out of so many mistakes people make, overfeeding is the most common. Also, feeding improper food to the fish is also a fault.

Even experienced aquarists may forget to switch on the power after they are done with the entire maintenance work. It simply means that fish will be deprived of some basic benefits they get from the technology like filtration, light, etc.

Other than this, stocking error, forgetting to change the water in the scheduled time or doing anything random unknowingly is pretty common.

All in all, there are plenty of things in which we can go dumb. You are going to mistakes but what matters most is learning from them and improving. So, just learn and move on!

Go easy on yourself. Do not worry. We are all human, and we all make mistakes.

  1. Problems beyond your control 

Apart from all the nine reasons we have discussed above, there are a few beyond our control. Sometimes, fish depart the life for different reasons that are beyond your control. For example, fish may have some congenital issues right from the birth. While a veterinarian can take a look and provide medications for a pup or kitten in such cases, checking on a fish is not possible.

So, if your fish has left for the heavenly abode and you are not able to find a satisfactory reason, it’s better to leave it. Maybe your fish was destined to live a short life, and you can’t do anything about it!

All you need is to evaluate your tank along with its management practices and make necessary changes wherever required.

Note: Most of the time, the aquarists breed fish in large stock. If the fish you buy always show some problematic symptoms or develop issues, maybe the problem is not with you but with your stock. In such cases, we suggest you change the shop from where you buy your fish. The issues could be with the supplier. The possibility of him not handling the stock properly in the store is also there. Such things are completely out of the control.


In this article, we have discussed the top 10 reasons why fish die. It will be helpful for all those who are learning fishkeeping or want to find out the reasons why their fish constantly die even after so much care.

Don’t get disheartened immediately, and don’t give up. Fishkeeping is one of those hobbies in which every day and with every fish you learn something new. Each mistake makes you better and helps you know more about fish and fish keeping. So, learn from your mistakes, do all the necessary research and get better.

If you are experiencing something unusual with the fish and none of the above reasons seem convincing, leave a comment below so that fellow aquarists may share their experiences with you and we may also try to help

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6 thoughts on “10 Most Common Reasons Why the Fish Die

  1. Vaishnavi Shewale says:

    A fish we bought killed about five other fishes and then one mourning all of the fish died altogether. Out of seventeen fish only three are alive now and one of them is unable to move much.

  2. Ravikant tiwari says:

    I am a begginner and purchased oscar,parrot fish but overnight they died. Installation guy was there who set up the tank.

  3. Linda Walker says:

    My 36 gallon tank has been good for about 2 years now but recently my fish are dying. I did a test for all of the important things like Ammonia, PH, etc. and all is good. My two large Angel fish seem okay but I have lost two small ones. One platty was huge and died in two days. I have reduced feeding. Not sure if I should do a 50% water change or what. I do weekly 15-20% water changes so not sure what is going on.

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