6 Reasons Your Fish Died After A Water Change

Last Updated on 2023-11-08

Did your fish die after a water change? It’s heartbreaking, but there are ways to prevent it from happening again. In this article, we’ll explore the common causes and provide tips on how to save a dying fish after a water change.

So keep reading to find out everything you need to know!

Key Takeaways:

  • Fish might die after a water change due to chlorine poisoning, temperature shock, ammonia boost, osmotic shock, pH fluctuations, or chemical contamination.
  • To prevent fish deaths after a water change, perform smaller, more frequent water changes, dechlorinate the water, maintain consistent temperature and pH, and preserve the filter’s bacteria colonies.
  • Techniques to save a dying fish include stopping feeding, dimming tank lights, increasing water aeration, using water conditioner, ammonia neutralizer, aquarium salt, and a carbon filter.
  • Addressed FAQ topics include reasons for immediate fish deaths after a water change, the necessity of water change after a fish dies, the stress of excessive water changes, and the impact of a 100% water change on fish survival.

Why Did Your Fish Die After A Water Change

If your fish died after a water change, there are a number of different things that may have caused it. In fact, it could have been any of the following: 

Chlorine Poisoning

One of the most common reasons fish die after a water change is due to chlorine poisoning. If you forget to dechlorinate your tank, then it’s going to cause massive damage to your fish’s gills and respiratory system and the rest of their body. Once the damage is too severe, your fish will perish.

Temperature Shock

Another common reason that fish die after a water change is due to temperature shock. If the water you added changed the temperature dramatically by either cooling it or heating it up, then it will also result in your fish dying.

Ammonia Boost

If you remove too much water from the tank, it can destroy a lot of the healthy bacteria colonies you’ve been harboring in your tank. When this happens, the ammonia in the tank will rise rapidly, which results in ammonia poisoning and, eventually, death.

Osmotic Shock

Did you know fish absorb minerals in the water through their skin to keep them healthy? The problem is when there aren’t enough minerals in the water, your fish will begin to go into osmotic shock.

The good news is if you add water to the tank that has been remineralized, your fish can also make a full recovery.

pH Fluctuations

And lastly, if the pH you add to your tank is too acidic or alkaline and dramatically changes the pH in the tank, this can also cause your fish to die rapidly. In each case, your fish will begin to show breathing difficulty, inflammation, and even skin burns if the water is too acidic or alkaline. 

Chemicals Got Into The Tank

Sometimes, it can be the case that chemicals have gotten into the tank. If the container you used to add new water was used for other things, then it may have been contaminated. Once this contamination enters the tank, it will cause carnage for your fish.

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How Do You Prevent Your Fish From Dying After A Water Change?

If you’ve already lost fish from a water change, the last thing you want is to lose more. Fortunately, there are plenty of things you can do to prevent death because of a water change.

So next time you’re performing a water change, make sure you do the following:

Preventive MeasuresDescription
Perform Smaller Water ChangesMake frequent, smaller water changes (e.g., 15% instead of 50%) to prevent drastic environmental changes and reduce the risk of ammonia and waste build-up.
Dechlorinate the WaterUse a dechlorinator or let the water sit for 2-5 days to remove harmful chemicals like chlorine and heavy metals before adding it to the tank.
Maintain Water TemperatureEnsure the water added is similar in temperature to the existing tank water; check using a thermometer to prevent temperature shock.
Check pH Before Adding WaterUse a pH testing kit to ensure the added water’s pH aligns with the tank’s pH; adjust using pH Up or pH Down to avoid pH shock in the fish.
Preserve Filter BacteriaKeep the filter bacteria alive by avoiding letting the filter dry out; rapidly return the filter media to the tank after cleaning.
Moderate Tank CleaningAvoid excessive tank cleaning to preserve healthy bacteria colonies in the tank, preventing ammonia spikes.
Clean the Filter RegularlyRegularly clean the tank filter to maintain water cleanliness and reduce the chance of sudden fish death due to water contamination.

Only Perform Small Water Changes

First of all, one of the best things you can do is perform smaller water changes more frequently. The number one reason to do this is that if you do make a mistake, the results are going to be much less dramatic if you’re changing 15% of the water, not 50%.

As well as this, performing smaller water changes more often is going to stop a buildup of ammonia and waste in your tank, which can also be fatal to fish.

And lastly, large water changes increase the chance of osmotic shock occurring. 

Dechlorinate The Water

And, of course, you should also make sure that your dechlorinating water before you add it to your fish tank. The easiest way to do this is with a dechlorinate-like API Stress Coat. However, if you can’t get your hands on this, there are a couple of other things you can do.

The first thing is you can try boiling the water. While this will remove the chlorine from the water, oftentimes, it won’t remove some of the harmful heavy metals in the water, which can build up in your fish over time.

And secondly, you can let the water sit out for 2-5 days. While two days is normally enough, depending on how chlorinated your water is, it can take up to 5 days to get rid of it all completely.

Maintain The Water Temperature

You should also make sure that the water you’re adding to your tank is similar in temperature to the water that is already in the tank. To do this is so simple, simply check the water with a thermometer before you add it to your tank.

Check The pH Of The Water Before Adding It

As well as checking the temperature of the water you’re going to add, you should also check the pH as well. To check the pH, you should use something like the API Master Test Kit. And if you notice the pH is too high or low, you can simply add API pH Up or API pH Down to the water to ensure pH shock doesn’t occur.

Don’t Let The Filter Dry Out

If the bacteria in your filter dies, then there will be nothing to prevent an ammonia spike from happening in your tank. And if you let your filter dry out, then this is exactly what will happen to the bacteria. 

So when you need to clean your filter media, make sure you’re adding it back to the tank as quickly as possible.

Don’t Overclean Your Tank

On a similar vein to not letting your filter dry out too much, if you’re overcleaning your tank, then you risk destroying the bacteria colonies you have. And if you do this, it will end up causing a spike in ammonia.

Clean The Filter

And lastly, you should make sure you’re cleaning the filter in your tank regularly as well. Cleaning the filter and making sure it isn’t getting clogged is paramount to keeping the water clean and reducing the chances of sudden death in your fish.

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How Do You Save A Fish That Is Dying After A Water Change?

If you have a fish in the tank which hasn’t died from a water change yet, but you think it might, then, fortunately, there are a few things you can do to save them. So try any of the following in the future.

Stop Feeding Your Fish

The first thing you should do is stop feeding your fish. If your fish is stressed, then food is not going to be the best thing for them, especially if their digestive system isn’t working as it should be. This can often result in constipation which is going to be even more stressful for your fish.

On top of this, any food that isn’t eaten by your fish is just going to rot in the tank causing the parameters in the water to become even worse. 

Turn The Lights Off

When your fish is stressed, one of the best things you can do is turn the light off for them. This way, they’re going to feel safer and more secure in the tank. Once they feel safe, they’ll be less stressed, and therefore their immune system will become stronger as well.

Aerate The Water

To aerate the water, all you need to do is increase the amount of surface agitation with your filter or add an airstone to the tank. And by aerating the water, you’re going to increase the amount of oxygen in the tank.

When there is more oxygen in your tank, it’s going to help reduce the amount of ammonia in the tank, which will be beneficial in saving your fish’s life.

Add Water Conditioner

If you’ve forgotten to add water conditioner to your tank, then there’s still time to do so. Add it as soon as possible near the filter, so it gets dispersed quickly. Depending on the size of your tank, you may also need to stir the water to make sure the chlorine is removed as quickly as possible.

A great water conditioner you should consider using is the API Stress Coat.

Use Ammonia Neutralizer

As well as using a water conditioner, if you think that the ammonia in your tank has become too high, and that’s what’s causing your fish to die, then you can also add an ammonia neutralizer to the tank. This will neutralize all the ammonia instantly while allowing it to still be consumed by the beneficial bacteria.

And when the ammonia is at zero, it’s going to give your fish the chance it needs to recuperate.

Try Adding Some Aquarium Salt

Aquarium salt is a great way to lower the amount of stress your fish is feeling, which can help keep them alive if they’re struggling. If you are going to use aquarium salt, make sure you’re using it sparingly and that you’re following the instructions beforehand.

Make Sure You’re Using A Carbon Filter

If chemicals have accidentally entered your tank, then a carbon filter is going to deal with them. As well as using a carbon filter, you should also make sure that you’re changing it regularly, as they often need replacing after a couple of weeks.

If you need some more insights on why your fish died after a water change, check out this video by Michael’s Fish Room!


Here are some frequently asked questions that people have about why their fish died after a water change!

Why Did Your Fish Die Immediately After A Water Change?

If your fish died immediately after a water change, then it’s most likely due to chlorine poisoning, temperature shock, or pH shock. Any of which will kill your fish a lot quicker than other causes.

Should You Change The Water After A Fish Dies?

Whenever your fish dies, it’s always a good idea to change a small amount of water to reduce the chance of ammonia building up as well as parasites and diseases overtaking your tank. You don’t have to perform a large water change. In fact, 10-20% is often sufficient!

Can Fish Survive A 100% Water Change?

It’s possible that fish can survive a 100% water change; however, it is extremely stressful for them. When you perform this sort of water change, the parameters will change completely, which can send your fish into shock. 

It’s much better to perform smaller water changes over a short period of time over one large one.

Can Fish Die From Too Many Water Changes?

If you’re performing too many water changes in a short period of time, then it’s going to cause extreme stress to your fish. This in itself can end up being fatal too them, and even when it’s not, it’s going to severely weaken their immune system.

So you should stick to changing your fish’s water every one to two weeks and only changes around 10-30%.


As you can see, there are a whole number of reasons your fish might have died after a water change. However, as long as you take steps to prevent it from happening again, then, fortunately, it won’t!

If you liked this article, make sure you check out the rest of the website! Otherwise, have a great day!


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