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Many natural and unnatural reasons will cause your fish to die.

Below we’ve provided a quick list of the most common reasons in no particular order.

  • Poor Water Parameters
  • Lack Of Beneficial Bacteria
  • Not Enough Oxygen, Suffocation
  • Water Temperature Too Hot or Too Cold
  • Water Pollution, Chemicals & Toxins
  • Overfeeding, Starvation, or Insufficient Diet
  • Old-Age
  • Not Enough Space
  • Loneliness
  • Aggressive Tank Mates (Predation)
  • Disease/Parasites
  • Fish Are Under Stress
  • Injury

Next, we’ll go into more detail about each reason, so you know how best to avoid them.

As we go through each question, you’ll notice that it’s never just one reason and usually multiple reasons your fish has died.

Let’s start with an obvious one.

Why Do Your Fish Die When You Take Them Out Of Water?

Intuitively, it is common knowledge that fish die when they are taken out of water.

But, do you know why?

Aquarium Maintenance Checklist [Free Guide]

A common misunderstanding is that fish don’t need oxygen to survive.

But it may surprise you to know that the reason fish die out of water is because they can’t breathe.

The blood vessels on their gills absorb oxygen from the water and circulate it through the respiratory system. And when taken out of the water, these gill arches dry out and collapse.

As a result, the blood vessels can’t absorb oxygen. The fish suffocates and dies because its body isn’t designed to live like that.

Another idea that may seem counterintuitive is that a fish can drown in water. This is another form of suffocation that comes from low oxygen levels in the fish tank.

From poor water quality to high population size, many reasons lead to a lack of oxygen.

Why Are Your Fish Dying In Your New Home Aquarium?

Getting a new tank can be extremely exciting. But nothing can put a damper on that excitement, like waking up the next day and finding out your fish died during the night.

If your fish keep dying, the culprit may be New Tank Syndrome.

As a beginner fish enthusiast, you can’t wait to bring home your new fish. But the most critical mistake you can make is purchasing fish before adequately setting up your aquarium and waiting.

Ask an expert why your new fish died, and a standard answer you’ll hear is “the tank hasn’t cycled.”

But what does that mean?

Fish food and fish poop have ammonia in them. Ammonia is toxic for fish, and if it builds up in the tank, it will bring down the water quality and kill your fish.

Ordinarily, nature intervenes, and you’ll see the formation of `good bacteria” in your aquarium that break down ammonia into nitrite. And then, a second bacteria break the harmful nitrite into harmless nitrate.

But your fish can die or become sick during the time it takes for these bacteria to develop. Many fish won’t acclimate and will die from stress.

The nitrogen cycle is the time it takes for these beneficial bacteria to form. Then and make the water safe for your fish.

This process mimics the natural water changes you would see in nature.

Regular water testing with a top-of-the-line aquarium water test kit will help you maintain high water quality.

Another reason your fish might die after you’ve added them to your new tank is they may have experienced rough travel during transport from halfway across the world.

If you want healthy fish, one tip is to buy ethically and from trusted local sellers.

Why Do Fish Die In A Bowl?

Everyone loves a fishbowl. It’s the ultimate centerpiece. But small fish aren’t an interior décor statement; they’re living, breathing creatures. And by putting them in a bowl, you’re practically sentencing them to death.

It sounds dramatic, but here’s why:

When you have too many fish in a tiny fish tank, there’s a lot of poop!

The good bacteria cannot keep up with the amount of ammonia being excreted, and the whole tank becomes toxic.

Then there’s the factor of oxygen depletion in the tank water, which causes your fish to die of suffocation.

Keeping one fish in a tiny bowl may seem fine but keeping too many fish in a small bowl isn’t a good idea.

If your fish keep dying, it could also mean you’re overdoing water changes. Ammonia and nitrite are harmful, but inconsistent water temperature and pH will kill your aquarium fish.

With a significant water change, make sure the water being added is the same temperature and pH as the water in the aquarium.

On the other hand, a large aquarium with a large filter can easily maintain water quality and keep fish healthy.

If you want to keep a bowl, you can keep one fish so the bacteria and ammonia levels even out.

Why Do Fish Die When You Change The Water?

Water changes are an essential yet potentially dangerous element of caring for fish.

There’s a lot of things that can go wrong, for example;

1. Chlorine

Chlorine is poisonous for fish species. If you don’t add appropriate amounts of de-chlorinator during the water change, it will kill the good bacteria in your aquarium.

With nothing to counteract the ammonia and nitrite, your fish will start dying of stress. That’s why the initial nitrogen cycle is so important.

To prevent toxic chemical buildup: regularly check the aquarium water ph levels and ammonia and nitrite quantities. Make small water changes as needed.

2. Temperature Changes

Many new fish keepers focus on the wrong issues. It doesn’t matter whether you’re adding warm water or cold water to your aquarium, but rather whether the water added is the same temperature as the existing aquarium water.

To prevent rapid temperature changes: control the external exposure of your aquarium to sunlight, wind, heating, or cooling vents.

Also, ensure the tank heater and filter are working correctly.

Additionally, fish like a familiar environment. Rapid water changes will put your fish under stress, and your fish may start showing signs of disease.

Typically, water changes should be done every 1-2 weeks, depending on the condition of your tank water.

To avoid shocking your fish: replace smaller amounts of water at a time and give them time to acclimate.

3. Toxins

Chemicals and toxins in fish tanks can also be a reason why your fish died.

Your fish tank is extremely sensitive.

That means reaching into the aquarium with lotion on your hands, spraying perfume near an uncovered tank, or even soap on the glass surface can kill your new fish.

To protect your fish from toxins: place a cover on top and replace your soap with a chemical-free, unscented alternative.

You don’t want to find out your fish died because you were wearing too much perfume.

Why Are Your Fish Dying All Of A Sudden?

Overall, if your aquarium fish are dying without any warning or signs of disease, it can mean that:

  • The water pH levels and other parameters in your fish tank weren’t suited to your fish species
  • Rapid temperature changes gave your fish stress, causing them to die
  • The fish experienced rough travel and were dying when you purchased them
  • lack of oxygen caused the fish to die
  • Toxins in your aquarium and poor water quality caused your fish to die
  • You did not buy healthy fish, but rather ones struggling with injury, disease, or old age
  • Fish died because there were high quantities of ammonia and nitrite in your fish tank and not enough good bacteria
  • Rapid changes in water temperature caused many fish to die of stress

To stop your fish from dying, you should:

  • Understand the water quality requirements for your aquarium fish
  • Replace tank water one cup at a time, with time in between so fish may adjust
  • Only purchase from reputable dealers and learn how to identify healthy fish
  • Choose your new aquarium, keeping in mind the size of the tank and filter, so fish have enough oxygen
  • Test your water filter and aquarium heater every week
  • Cover your tank to prevent toxins and chemical influences from entering your fish tank
  • Maintain good water parameters
  • Regularly test the water quality, so there is no ammonia buildup in your aquarium
  • Ensure ample good bacteria are present to break down ammonia by cycling your tank
  • Prioritize aquarium maintenance and clean your tanks every few weeks, so poop and fish food do not start polluting the aquarium
  • Learn how to identify signs of fish diseases and the presence of harmful parasites

Why Are Your Fish Dying One At A Time?

You bring home a few freshwater fish. Then one fish died, and you attribute it to disease.

The next day you lose another.

Now all your fish are dying one at a time, and you don’t know what’s wrong.

Here are a few things that might be causing your fish to die one by one.

Disease or Parasites

The most straightforward answer is parasites. If they’ve entered your aquarium, your fish may die within weeks.

To save your aquarium fish, you must identify the parasites or diseases. Then you can look for possible cures and medicine.

Poor Water Parameters

Water quality refers to the ammonia, nitrate, and nitrite amounts in the fish tank.

As previously mentioned, keeping a balance between these three chemicals is the key to keeping fish healthy.

Conducting a fishless nitrogen cycle before adding your new fish will boost the production of good bacteria and harmless nitrate so your fish won’t spend torturous weeks adjusting to the water change.

If you filter the water properly and encourage bacteria, you’ll keep your aquarium fish happy.

Overfeeding

Fish food also comes with ammonia. If it collects in your fish tank, it will unbalance the water chemistry.

Fish often die because their owners don’t clean their tanks for weeks.

This allows the spread of diseases, buildup of ammonia, depletion of air, and even encourages the entry of parasites.

Don’t dump excessive amounts of fish food into your tanks because it could kill your fish. Stick to a strict schedule with your new fish, and make sure you remove any food that collects at the bottom of the tank.

Aquarium Maintenance

Aquarium maintenance needs to occur every few weeks.

That means compulsory water changes once a week. Replacing a standard 10 to 20 percent water is an excellent way to filter and clean the tank.

This keeps ammonia levels in control without upsetting the bacteria balance.

Also, make sure when you wipe the aquarium glass, don’t use chemical cleaning agents, and test water quality regularly.

If fish poop or food has been collecting at the bottom of the tank for weeks, clean it out.

Mixing Incompatible Tank Mates

You’ve run multiple tank tests, checked the water quality, filter, and tank heater, and there’s nothing wrong.

Now you find yourself asking, “Why are my fish dying?”

One reason all your fish are dying might be that you’ve created an underwater war zone.

That happens when you mix incompatible fish.

If you’re putting aggressive players in your aquarium, they will kill one another, and you’re going to wake up to dead fish.

Chinese algae eaters and male betta are the wrong choice of fish to mix. Put a blue gourami with other aquarium fish, and it will kill any intruders in its territory.

You’ll quickly realize something is wrong when you start seeing dead fish.

The solution: Do your research and look for good tank mates before you buy your fish or risk buying fish that will kill each other.

Why Do Fish Die In Water That Is Too Warm Or Too Cold?

Rapid changes in water temperature can kill aquarium fish, especially if you put warm water fish in cold water.

Fish like a specific water temperature, and anything outside that range will cause them stress.

There’s also fish behavior to account for.

Your aquarium fish will become more active in warm water, leading to a shortage of oxygen and food. If the tank temperature exceeds 90F (32C), it can kill your fish.

However, adding cold water to your tank could also kill your fish by reducing their metabolism and immunity, thereby making them more vulnerable to parasites.

The stress from cold shock may be why your fish are dying.

To prevent this, pay attention to signs that show something is wrong. These include gasping fish, increased algae, rapid swimming, or bulging eyes.

Ideal tank temperatures for:

  • Common tropical fish – 75 to 78F
  • Common goldfish – 62 to 68F
  • Specific types of tropical fish (Rams, Discus, Clown Loaches) – 74 to 84F

Should You Change Your Water After Your Fish Die?

If your fish died, there are specific steps you need to take to make your fish tank ready for new inhabitants.

Dead fish quickly begin to rot and pollute the tank. Remove them immediately to prevent the spread of diseases.

Also, keep in mind that the remaining healthy fish may begin eating the dead fish the very next day.

This will allow parasites, bacteria, and diseases to spread in your fish tank. Hence, time is of the essence.

Your immediate concern is not a water change but rather to identify why the fish died.

How can you tell what’s wrong with your tank fish?

A bloated body means the fish died of infection, while split fins could mean a fight with other species in the fish tank.

Finally, test the water quality for ph levels, excess quantities of ammonia and nitrite, water temperature, or oxygen depletion.

If the fish died because of these reasons, conduct an emergency water change.

When there is a problem with the water quality in your fish tank, you can go from one dead fish to multiple within weeks.

How Do You Stop Your Fish From Dying?

No one wants to wake up to see dead fish floating in their tank.

But, there’s nothing more depressing than not knowing what’s wrong with your fish.

When you get a new aquarium, you need to be extremely vigilant in the weeks after.

When the fish are adjusting to their new home, the most problems will arise in a new tank.

And if one fish died, you need to make sure there isn’t a problem in the tank.

There are a few steps you can take to stop your fish from dying.

  1. Regularly check the water quality of your fish tank and keep tabs on the ammonia, nitrate, and nitrite amounts.
  2. When you buy new fish, research the diseases and parasites it is most susceptible to. Learn how to identify signs of diseases in your tank. You need to know when something is wrong.
  3. Make sure you cycle your new aquarium before adding any fish.
  4. Regularly check the water temperature and make sure it’s within the range for your fish species.
  5. Learn how to conduct a proper water change while making sure there are no significant jumps in water temperature that could stress the fish.
  6. The aquarium size should suit the number of fish you have so they have enough room and oxygen in the tank.
  7. Follow a feeding schedule, so you don’t overfeed the fish. Also, clean any uneaten food, so it doesn’t settle at the bottom of the tank and increase ammonia levels.

Overall, the main reason fish die is poor water quality so test your water as often as you can and stay on top of any emerging tank problems.

Video: Why Do Fish Die?

Here is a video from KG Tropicals that goes over the top 10 causes of fish death.

FAQ:

Why Do Fish Swim Upside Down Before They Die?

Your fish will start to swim upside down because the air and other gases start to become trapped in its belly.

Since the top part of the fish has more bone and muscle, it flips upside down.

Why Do Fish Float When They Die?

Fish float when they die because the air remains trapped in its body, causing them to float when they die.

That’s why pet owners often go to sleep with a full tank of fish and wake up the next day to see their pets floating on the top of the fish tank.

Why Do Saltwater Fish Die When Placed In Freshwater?

A saltwater fish will die if you place it in a freshwater aquarium because its body has a high salt concentration.

You probably studied the process of osmosis in your eighth-grade science class.

Using the same principles, the fish’s body will absorb water for a few weeks until the fish bloats and dies.

References