Top 7 Reasons Fish Keep Dying But Water Is Fine (+Solution!)

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For anybody fond of keeping fish, having them dying in the aquarium can be a nightmare. Finding out why they’re dying can be even more challenging if the water in the tank is fine.

The top reasons why fish die even when water conditions are good include stress, diseases, overfeeding, and water temperature fluctuation. Other reasons can be that the tank is overcrowded, or has become too small, or other parts of it are not cleaned regularly.

Read on to find out more about why your fish might be dying despite your best efforts to keep their water clean. Happy reading!


1. The Fish Have Contracted A Disease/Infection

Fish dying from disease or infections is a more common occurrence than you might imagine. More worrisome is the fact that it is often hard to notice if the fish are not well.

They might be sick with some disease, and you might not even know what’s going on.

Disease or infections can be introduced whenever a new fish is added to the tank. There’s a chance that the new fish you’ve introduced to your tank may already be carrying some disease or parasite.

This can have fatal consequences for the fish that are already in the tank.

Harmful organisms can also enter the tank if you’re using some tools and instruments to clean and maintain it that aren’t properly sterilized.


Keep any new plants or fish in a separate tank for a while before adding them to your aquarium. I recommend you to keep a separate a spare tank like this one available on Amazon as your quarantine tank.

This way, you can find out if any of these new additions are carrying diseases.

Also, sterilize any tools or nets that you use for cleaning and maintenance.

What Kinds Of Diseases And Ailments Can Fish Get?

Similar to other animals, fish are prone to getting sick. Here is a list of some of those diseases that commonly affect aquarium fish.

  • Red pest disease. This condition causes red patches to appear on the body of the fish. It can also lead to tail and fin rot. This disease is caused by toxic water conditions.
  • Dropsy. This ailment in fish occurs when fluid builds up inside their bodies, making them bloat. It can be caused by many factors including stress and sudden changes in water quality or temperature.
  • Velvet disease. This disease is caused by parasites in aquarium fish. It can make the fish lose appetite and weight.

2. The Fish Are Stressed

This is the most common cause of death for fish in an aquarium. Just like stress has negative impacts on our health, it can also affect fish — and is often fatal for them.

Fish are sensitive creatures and need lots of space to swim around freely, without feeling threatened or suffocated. Certain factors can cause fish to become uncomfortable and eventually, stressed.

Here’s a list of some such factors.

  • If the tank is too well-lit and more lighting than necessary is used.
  • If the tank is very small and has little space for fish to swim.
  • In case a new species is introduced, the existing species may feel threatened.


Make sure the water and temperature conditions in the tank are optimal for the fish. There should also be lots of gravel and aquatic plants that serve as places where fish can hide or isolate.

Avoid adding any new species that the old ones might not adjust well to.

How To Find Out If Your Fish Are Stressed?

Being stressed is bad for fish. But how can you tell if your fish are stressed? These are a few symptoms to help you find out.

  • visible changes in the appearance of the fish
  • hiding in the corners or in aquatic plants
  • decrease in appetite    
  • not recovering from disease, even after a while
  • swimming throughout the tank at speed (in desperation)

If your fish exhibit any of these behaviors, chances are they might be stressed about something.

3. The Fish Are Overfed

Like underfeeding fish is a problem, overfeeding them can be dangerous too. Overfeeding fish can be harmful in multiple ways.

Firstly, fish that eat more than they need are likely to get bloated and have digestive problems. This can cause many problems for them and may eventually kill them.

Secondly, if you feed the fish too much, more than they need, then there will be lots of it that won’t get eaten and can affect the nutrient balance of the water. 

The water can become toxic and thus kill fish. Usually, a small amount of leftover food isn’t a problem but large amounts of it can be quite dangerous.


To prevent overfeeding, give your fish the amount of food that they can consume within a few minutes. Any more than that, and you’ll risk overfeeding them. More importantly, remove any leftover food when you’re cleaning the tank.

I think you’ll also find it helpful to have a look at my complete fish fasting guide.

4. The Tank Is Overcrowded By Fish

If your water tank or aquarium is overcrowded with fish, it can be dangerous to their health.

Overcrowding likely occurs when you’re new to fishkeeping and want to add as many fish as you can to your aquarium.

While the aquarium that is teeming with fish might look great, overcrowding can wreak havoc on its ecosystem.

Overcrowding can cause the water to become toxic since now the bacteria in the water are unable to break down increased quantities of waste. 

Besides making the water toxic, overcrowding can also make the fish stressed since they have less space to move freely.


Avoid adding new fish if the tank isn’t big enough to accommodate them. Read my guide on how many fish you can add at once.

A particular sized tank can allow only a certain number of fish to live comfortably. There simply isn’t more room to introduce any new ones once that capacity is reached.

5. The Tank Is Too Small

If your tank is too small, it can increase the risk of fish dying. The reason for this is that the fish can become stressed due to the lack of available space for swimming freely. 

Fish are quite sensitive to their surroundings and a lack of space can cause them great distress.

A common scenario is when you buy juvenile fish and choose the aquarium according to their sizes at that time.

But as the fish grow larger, the tank is no longer big enough to accommodate them comfortably. For this reason, the tank needs to be replaced.


Evaluate if it is time for you to get a larger tank based on the number of fish and their sizes. Juvenile fish grow quickly and you might need to replace your tank more than once.

Tips For Choosing The Correct Tank Size

If you’re new to fishkeeping and are wondering how big your tank should be, there are a few important things you must consider.

First, decide how many fish you’re going to buy and how many you plan to add in the future. The more fish you’ll be having in your tank, the bigger it should be.

Moreover, if you’re planning on getting juvenile fish for your tank, it is a good idea to go for a bigger tank since they’ll grow eventually and require more space.

6. The Temperature Keeps Fluctuating 

So, the water in your tank is good and healthy, but is that enough? The temperature of the water is also important.

Although some species fare better than others in above-average or below-average temperatures, most species can survive in a very specific range of temperatures.

Betta Fish75-80 Degrees F
Goldfish68-74 Degrees F
Angelfish78-84 Degrees F
Tiger Barb77-82 Degrees F

Fish are very sensitive to the water temperature as well as to any fluctuations or changes in it. You must ensure the water is always at optimum temperatures.


Move your tank away from direct sunlight and keep it away from any sources of cold winds. Regularly check the heater to ensure it’s working properly.

7. The Tank Is Not Maintained Regularly

Routine cleaning and maintenance of your aquarium don’t solely entail changing the water. Even if the water is clean, many other parts of the tank might not be and that can negatively impact the health of your fish.

Fish need a clean and healthy environment to thrive. Not cleaning your tank regularly can cause that environment to become polluted and unlivable for them. 

And this can lead to the death of fish in large numbers.


Create a maintenance schedule so that you can thoroughly clean the tank at regular intervals. Sticking to this schedule is very important. Skipping cleaning duties can quickly make the tank environment unhealthy for your fish.

How To Clean And Maintain Your Tank Properly?

Cleaning your tank or aquarium regularly is really important in ensuring the well-being of your fish. Here are some steps to help you get started with it.

  1. First, measure the water quality using a water test kit to check the amount of ammonia in the water.
  2. Use an algae scraper to clean the inner walls of the tank.
  3. Remove those decorations that are really dirty. Sterilize them and put them back.
  4. Use an aquarium siphon to remove debris and fish waste. Also, remove roughly one-third of the tank’s water.
  5. Thoroughly clean the aquarium filter.
  6. Add clean, fresh water to the tank. The water should be at the correct temperature.

How Often Should You Change The Tank Water?

Changing the tank water is a necessary part of your aquarium’s regular maintenance. It is necessary to drain about one-third of the water and replace it with clean, fresh water.

So, how often should you change your tank water? 

The answer depends on the number of fish in your tank. If you have lots of fish, changing the water once every week is a good idea. If you have less fish, changing it once every ten days or two weeks would be fine.

Final Thoughts

To sum it up, there are certain reasons why fish might die even if the water in the aquarium is clean and fresh. But for every reason, there are also some solutions that you can implement to diagnose the problem and stop it from happening.

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