10 Reasons Your Fish Keep Dying (& What To Do)

Discovering that your fish are dying in your aquarium can be a disheartening experience, to say the least. Fish are beautiful, delicate creatures, and we quickly grow fond of them. There are a plethora of reasons why your fish keep dying, and finding out solutions to prevent that is important.

Your fish may be dying due to several reasons. These include stress, overfeeding, overcrowding, toxic water conditions, fluctuating water temperatures, the introduction of incompatible species, the onset of disease, and the death of beneficial bacteria in the water.

Read on to find out more about these reasons and also discover their solutions. Happy reading!

Why Do My Fish Keep Dying

There are a lot of reasons why your fish keep dying, depending on a range of environmental and other factors. Here are 10 of them.

1. Stress

Stress is one of the most common causes of fish death. Just like humans, fish can get stressed, too, and it can negatively impact their health. It can even prove to be fatal if the fish remains stressed for an extended period. Fish can become stressed for a multitude of reasons. Identifying that problem and fixing it is really important for the well-being of your fish.

When fish get stressed, they exhibit certain behaviors that you can watch out for. They usually swim all around the tank, particularly near the walls, and try to hide in certain parts of the tank.

2. Overfeeding

Like feeding too little can prove to be fatal, so can overfeeding. Overfeeding your fish can make them sick, and prolonged symptoms can even be fatal for them. This usually happens with those who are looking after fish for the first time. In an effort to feed them well, they overfeed them. It can make the bodies of the fish bloated and may kill them.

Another problem associated with overfeeding is that the excess feed can turn the water toxic since it cannot be broken down by the bacteria.

3. The Tank Is Overcrowded

If your fish keep dying in large numbers, overcrowding your tank might be to blame. Fish are extremely sensitive creatures when it comes to space, and they need large spaces to swim around. When space is limited due to other fish being present, a fish might get quite stressed out and seek hiding places to isolate itself. It may also become frantic and try to escape the aquarium by swimming near the tank’s walls.

Many tropical fish swimming in a dirty aquarium with murky water.

4. Bad Water Conditions

Toxic water conditions are one of the biggest reasons behind mass deaths in fish. As we mentioned previously, fish are quite sensitive and only thrive in optimum water conditions. Toxic water conditions can cover a range of problems. These include incorrect pH levels, excess feed in the water, residue from gravel, and many other factors that can make the water unsuitable for supporting fish. Problems with your tank’s filtration system and the addition of potentially harmful chemicals can also make the water toxic.

5. Lack of Tank Maintenance

Cleaning and maintaining your tank regularly is very important in keeping your fish alive. Beginners often tend to overlook this and instead focus solely on feeding the fish. However, it should not be ignored as a dirty tank can severely impact the water quality, leading to fish deaths. Regular maintenance of a fish tank encompasses a big list of activities that need to be done. Making a cleaning routine and setting up reminders can help you perform tank maintenance regularly. In later sections, we’ll discuss more on how you should clean your tank.

6. Disease and Infections

This one is obvious. Diseases and infections are responsible for the deaths of fish, as you’d imagine. Just like humans, fish can get numerous diseases and infections, which, if untreated, can ultimately result in their deaths. There are two major factors that can cause diseases: toxic water and stress. Here is a list of some of the diseases that fish can get. 

  • Dropsy. It is the accumulation of fluid inside the bodies of fish.
  • Fin rot. Fin rot can occur due to poor water conditions or altercations with other fish.
  • Velvet disease. In this condition, fish can get red or rust-colored spots all over their body. Again, this happens due to bad water quality.

7. Introducing Incompatible Species

If the water in your tank is good, you clean the tank regularly, and the fish don’t have any disease, you might be wondering why your fish still keep dying. A probable reason might be that your tank has species of fish that aren’t compatible with each other. If you plan on having different species of fish in your tank, researching beforehand which species are compatible with each other is a good idea. Some species of fish simply can’t live with another species. The existing fish in your tank might be meek and peaceful, but if you introduce a larger and violent species, the existing fish will likely become stressed.

8. The Fish Tank Is Too Small

If your fish tank is too small, it might be the reason some of your fish are dying. A small tank means less space for each fish, which in turn can lead to stress, uneasiness, and discomfort. Since fish like to have lots of space to themselves and also frequently seek out hiding places, a tank that is big enough for them to swim around is necessary. Many beginners make the mistake of choosing a small tank and then adding fish to it over time, and so it becomes inadequate to accommodate them.

9. Fluctuations in Water Temperature

Besides being sensitive to the quality of water in the tank, fish are also pretty sensitive to the water temperature. Most fish thrive on only a specific range of temperatures and when the water temperature falls out of this range, they can become stressed and sick. Fish also don’t adjust well to frequent changes in the temperature, so maintaining a steady water temperature is of paramount importance.

10. Absence of Beneficial Bacteria

Beneficial bacteria are a very important part of an aquarium’s ecosystem. They establish and regulate the nutrient balance in the tank, break down waste particles and thus play a very important role in keeping your fish healthy. They are the organisms that prevent the water from becoming toxic and eventually killing your fish. Improper tank management and maintenance practices by beginners are usually the causes of these bacteria dying. Read on to find out how you can prevent these bacteria from dying.

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How Do I Stop My Fish From Dying?

Now that we’ve looked at the top 10 reasons why your fish might keep on dying, it’s time to look at solutions to these problems so that you can prevent further deaths.

1. Create a Maintenance Routine and Stick To It

One of the best ways you can stop your fish from dying is by creating a regular maintenance routine and sticking to it. Having a clean tank and clean water eliminates many of the health risks that can affect fish. On average, cleaning the tank thoroughly once every two weeks should be enough for most medium-sized aquariums. Here are some of the things you should do as part of the maintenance routine.

  • Replace about one-third of the tank’s water.
  • Use an algae scraper to scrub off algae from the inside walls of the tank.
  • Clean the tank’s filter.
  • Siphon gravel.
  • Use water test kits to monitor the water quality.

2. Monitor Water Quality

Constantly monitoring your water quality is one of the best ways to prevent fish deaths and ensure they thrive. This can be done using a water test kit, in which you can test parameters such as the pH, amount of ammonia, salinity, and the presence of metals and minerals. Try to maintain these parameters if they are fine, and don’t add any substances to the water that might change them. If your fish start dying, you’ll have to act immediately and replace your tank water.

3. Quarantine Any New Fish or Plants in a Separate Tank

The best way to prevent fish deaths due to diseases and infections is to make sure they aren’t introduced to the water in the first place. Identifying the source of a disease can be quite difficult, but most diseases are introduced by new fish and aquatic plants and can have devastating effects on a healthy fish tank. To prevent this, quarantine any new fish or aquatic plant you want to add to your aquarium in a separate tank for about a week. This will allow you to figure out whether the new creatures are carrying any disease.

4. Feed Only the Required Amount of Food

For beginners, it can often be hard to judge what amount of feed is adequate for fish, and they usually add more than is required. A common misconception among them is that having excess food in the tank is harmless. But as we discussed earlier, it can cause mass fish deaths. Overfeeding is a major cause of fish deaths, and taking care while feeding your fish is quite important in preventing that. Fish usually only need an amount of food that they can eat up in a few minutes. Anything more than that will lead to overfeeding.

5. Choose Your Tank Carefully

Choosing an appropriate tank size is a very important decision and must not be taken carelessly or hastily. The size of your tank does have an impact on the quality of life of your fish, and choosing the wrong size can have harmful consequences. While a bigger than necessary tank is not a big problem, choosing a smaller tank can be a problem if you plan on adding lots of fish to it. It can lead to overcrowding and stress in fish. So, before you choose your tank size, decide how many fish you plan on putting inside it.

6. Maintain a Constant Water Temperature

Maintaining a constant water temperature is crucial in preventing mass fish deaths since fish are quite sensitive to temperature fluctuations and don’t adapt well to them. To do that, you might need to get a heater for your aquarium. Heaters are especially necessary if you’re keeping tropical fish. And making sure that the heater is working fine is also important. Besides that, make sure the aquarium isn’t exposed to direct sunlight or cold winds. Hence avoid placing it near large windows.

7. Prevent Beneficial Bacteria From Dying

We’ve discussed before that beneficial bacteria play a vital role in maintaining the nutrient balance in the fish tank and keeping the water healthy. When they die, your fish die. But how can you prevent them from dying? One of the ways you can do that is by making sure you add a dechlorinating substance to the water. It will prevent chlorine, a major killer of beneficial bacteria, from doing its job.

8. Research Compatibility Before Buying New Species

Introducing new species of fish that aren’t compatible with the existing species in your tank is the perfect recipe for disaster. We mentioned before how some species just don’t get on with other ones. This can cause stress and eventually death. So, before you introduce a new species, do your research properly and find out which species can peacefully coexist with the ones in your aquarium. There are lots of research articles and guides that you can refer to, and aquarium or fish sellers can also guide you on the matter.

9. Don’t Add Harmful Chemicals

Adding harmful chemicals can lead to the deaths of many fish in your tank. This usually happens when beginners add chemicals to improve the water quality, which then adversely affects the fish. Besides that, avoid using deodorants, air fresheners, colognes, and insect sprays near your aquarium since they can get absorbed into the water and potentially kill your fish. The most effective way to stop this from happening is to get a good quality aquarium cover that prevents harmful toxins from entering the water.

10. Add Decorations and Aquatic Plants 

Preventing your fish from getting stressed is one of the best, if not the best way to prevent and stop mass deaths. Fish like to swim around freely and regularly like to hide in hiding spots around the tank. Like humans, they like to have some alone time. This helps reduce their stress levels and is good for their overall well-being. You can place various decorations, such as pieces of wood and some aquatic plants to provide lots of hiding spots for your fish. 

Frequently Asked Questions (FAQs)

Here, find out answers to some other related questions you might have.

Should I Change The Water If A Fish Dies?

Having a dead fish in your tank can be an incredibly stressful and disheartening experience. You might be feeling confused and lost and may not know what to do next. So, should you change the water if a fish dies in your tank? The first thing to do is to immediately remove the fish as it can start polluting the water. Next, test ammonia levels in the tank since dead fish release a lot of ammonia. Depending on your results, you might need to replace some of the tank’s water. Have a look at this guide for more details.

Why Do My Fish Keep Dying When I Change The Water

A water change may lead to the deaths of many fish in your tank. There are many reasons for this. Firstly, the new water might not have the beneficial bacteria in it that are essential for supporting fish. Moreover, changing a large amount of the tank’s water can also change the chemical and nutrient balance of the water, and the fish might have trouble adjusting to it. So, in order to avoid that, the water must be changed regularly, but in small amounts, and any big changes or additions to the water should be avoided. Take a look at this article if you want to learn more about changing your tank’s water the right way.

Why Do My Fish Keep Dying In My Pond

Fish in your pond might be dying due to a number of reasons. Identifying and diagnosing those reasons and then acting accordingly to stop the deaths is really important. Here are a few reasons your fish are dying in your pond.

  • overcrowding
  • overfeeding
  • a lot of algae in the pond
  • spread of diseases and other medical ailments among the fish

This guide goes into more detail about the causes of fish deaths in a pond.


Watching your fish die is quite an unpleasant experience — one that nobody wants to go through. If that happens to you, don’t lose hope and act fast to determine what is causing those deaths. Identifying the problem correctly means you’re halfway there to solving it. There are several reasons fish die, from bad water conditions to the introduction of incompatible species. In this guide, we’ve mentioned the top 10 reasons fish die and have also provided solutions to help you prevent it from happening again.