Fish Dying after A Water Change [How To Save]

Dying fish after a water change is very common, particularly for beginners. But why do fish die after a water change when you change the water for the betterment of the fish? Isn’t it an irony?

Fish die after a water change mostly because of changing the water in the wrong way. Improper water change can stress your fish and lead them to die. However, identifying the symptoms earlier will help take measures to save the dying fish.

Fish display various ominous signs immediately after a water change. You need to spot those and should act accordingly to save your dying fish.

Why are my fish dying after I changed the water? [Possible Reason]

Dead catfish ancistrus fish at the bottom of the aquarium
Dead catfish Ancistrus fish at the bottom of the aquarium

Poor water condition slowly weakens the immune system, and they become susceptible to disease.

Regular change of water in a fish tank is a great way to maintain healthy water parameters.

Yet, changing the water sometimes brings unexpected fish death.

But why are your fish die after a water change? There are quite a few reasons. However, the root cause is the drastic change in fish tank water parameters.

Super clean water here is responsible for fish to die where they were doing fairly well in dirty water.

The fish are capable of handling the gradual change over time. Nevertheless, changing water may alter the water chemistry in an extensive way. Fish can’t tolerate that, and they die.

In the below section, I’ve discussed the four major reasons for fish dying following a water change.

Quick Shift of Nitrate Ion

A long term exposure to high nitrate levels is harmful to fish. Consequently, one of the major reasons for water change is to reduce nitrate from the water.

However, a sudden drop in nitrate level following a water change can lead to an Osmotic shock for your fish.

Nitrates are ions (NO3-)  and fish has the capability to regulate its body to control the ions intake.

Ideally, nitrate levels in a freshwater aquarium should be kept below 20 mg/L. However, any changes should occur slowly, only removing less than 50 mg/L of the Nitrate per day.


However, an osmotic shock is a phenomenon when fish can’t regulate the ion absorption rate.

It forces the fish to absorb more fluid in their bodies resulting in swelling of the organs similar to dropsy.

Swollen organs put pressure on the swim bladder and the fish can’t control their balance while swimming. They often swim erratically, or upside down.

Sick goldfish swims upside down in aquarium.
Sick goldfish swims upside down in the aquarium.

The water Was Too Cold

A sudden change in water temperature is detrimental for fish as they experience temperature shock.

If you perform a water change without preheating the water a sudden temperature fluctuation may happen. This causes your fish to undergo temperature shock.

Fish are cold-blooded animals, unlike ion absorption, they can’t regulate their body temperature. Therefore, the recommended practice is to use water that matched the aquarium temperature.

Fish that suffers from a temperature shock stop moving normally, lose their color, and sometimes may die instantly.

But can fish recover from water shock?

Yes, fish can recover from a water shock, but it has a long-term effect. Their immune system weakens, and they become vulnerable to various diseases.

The outbreak of parasitic disease Ich is common when fish are exposed to temperature fluctuations. Ich is difficult to treat and often leads to fish death.

Tap Water Was Added Without Conditioning

Tap water contains chlorine and chloramine. Those chemicals are deadly for fish.

Therefore, you should condition the water before adding it to the aquarium. The most common way to condition the water is to use a de-chlorinator.

If you didn’t use a water conditioner, your fish will perish. A water conditioner helps to remove chlorine and chloramine from the water.

It also detoxifies ammonia and nitrite, and some even restore the protective slime coat of the fish.

But can you add a water conditioner while the fish are in the tank?

It’s possible to add a water conditioner when fish are in the tank. Seasoned aquarists who are experienced often add tap water and use the conditioner simultaneously.

In the below table, I’ve listed a few of the most used water conditioners for water change.

Name Best For Dose Price Where To Buy
Seachem Prime Removes chlorine & Chloramine. Detoxify ammonia, nitrite, and heavy metals. 5 ml for 50 gallons 20$ for 500 ml Available On Amazon
Seachem Safe It’s a dry version of Shechem Prime, and even more concentrated 1.25 g per 300 gallons 12$ for 250 g Available On Amazon
API stress Coat Removes chlorine, and chloramine and detoxifies heavy metals. Reduces stress on fish, and replaces slime coat on fish. 15 ml per 30 gallons 7.8$ for 16 oz Available On Amazon

Nitrifying Bacteria Are Lost

Did you replace the filter media, or cleaned the tank thoroughly on the same day?

Nitrifying bacteria grow colonies in filter media, in the substrates, and basically in every place in the aquarium.

Aquarium water also contains bacteria. So, you can lose some of the good bacteria through water change.

However, a significant portion of bacteria live in the filter media and substrates. So, changing the water, and cleaning the filter media, and tank on the same day may destroy the nitrogen cycle.

The loss of bacteria will result in an ammonia and nitrite spike since there are not enough bacteria present to break down the waste.

One dead fish in a fish tank with blue water
One dead fish in a fish tank with blue water

How Do You Know Fish Are Dying After A Water Change?

If the water change is not appropriate your fish may show various symptoms.

The notable signs that fish exhibit after an improper water change are erratic swimming, gasping at the water surface, and laying at the bottom.

Let’s dig down a bit more details on each of the symptoms.

Fish Laying At The Bottom

Other than a bottom-dwelling fish if fish lay at the bottom of the tank, it’s a matter of concern.

It’s more meaningful if the fish was behaving normally before the water change.

But, what might be the cause of this abnormal fish behavior? Some of the possible reasons are:

  • The nitrogen cycle may be hindered due to the loss of good bacteria. In the absence of sufficient bacteria, toxic substances build quickly. That makes the fish become lethargic and weak.
  • Temperature shock is another possible cause. Fish that experience a temperature shock often lay at the bottom.

I have an article on all the possible reasons why betta fish is laying at the bottom. I encourage you to read that if you are a betta owner. It applies also to other tropical fish.

Fish at top of tank after water change

Fish can also move at the top of the tank after a water change.

Ammonia and nitrite poisoning damage the fish’s respiratory system and also drives away dissolved oxygen.

Sick goldfish swims upside down in aquarium.
Sick goldfish

It happens when you lose a large portion of bacteria due to water change. That causes breathing difficulties, and fish gasping at the surface for oxygen.

Fish Swimming Erratically After A Water Change

Fish swimming erratically after a water change is also very common.

According to the earlier discussion, the large shift of nitrate is the main cause of this.

goldfish upside down
goldfish upside down

You can prevent that in two ways:

Change Water Before Nitrate Level Goes Too High

Change 10-25% of the water every week, so that nitrate can’t get too high. Although, how often you’ll change the water depends on many factors, as a general rule weekly change is a good practice.

Test the water before changing the water

Fish can tolerate a change of 50 mg/L per day. So, if you test the water and found that the nitrate level is 80 mg/L. In that case, you can bring down the nitrate to 30 mg/L maximum.

Now calculate the % of the water you can change. Here, if you change 50% of the water, the nitrate level will be 40 mg/L.

How do you save a dying fish after a water change?

The trick to saving the dying fish after a water change is to restore the original situation in the tank.

However, be careful don’t do this rapidly as it may worsen the situation.

  • Adjust Temperature: If the temperature you need to adjust do this slowly. Gradually achieve the desired temperature in the tank.
  • Check pH: A change in pH is also harmful. Too acidic or too alkaline water is not good for the fish. Check the pH and modify it accordingly. To help you out, I have two different articles both on how to increase or decrease pH levels in the tank. I encourage you to read that if you need to adjust the pH.
  • Test Ammonia & Nitrite: Test the water, and check if there is high ammonia or nitrite. If the ammonia is high an ammonia filter or ammonia remover might be quite handy as a quick relief.
  • Add Oxygen: Measure the oxygen concentration, and check if the saturation is more than 70%. If the dissolved oxygen is low adding an air pump will help.
  • Nitrifying Bacteria: If the fish suffering is due to the loss of bacteria, you can add some bacteria starters in the water. Although it’ll take some time to restore bacteria, it will help fish deal with the ammonia and nitrite spike.
  • Hospital Tank: Transferring fish to the hospital tank having the right water parameters is another option. However, still, some of the fish may die.
a dead gold fish on a netting
a dead goldfish on a net

But sometimes whatever you do might not be enough to save a dying fish. In that case, you have to remove the fish immediately and dispose of the fish ethically, and in an eco-friendly way.

Pro Tips to Avoid Fish Dying After The Water Change

Here are some pro tips to avoid fish dying after the water change.

  • Always perform the water change on schedule or after testing the water.
  • Never clean the tank or change the filter media on the same day as the water change.
  • Always condition the water with a de-chlorinator.
  • Don’t change the water more than 50% at a time.
  • Don’t transfer the fish to another tank while changing water because it may double up the fish’s stress.
  • Always use water that is similar to the tank temperature.

For more tips and guidelines please read my article on how to change fish tank water without killing fish.

Bottom Line

Change the water on a regular basis. That will keep the fish healthy as well as there will be no such incidents of fish death following a water change.

Although some aquarists never change the water in their tank. That’s because they maintain a heavily planted tank with low fish density.

Plants help clear up the nitrate. However, if you have to change the water never skip the vital steps of testing and preparing the new water.

Also, never change more than 50% of water at a time, and don’t combine the tank or filter cleaning on the same day of the water change.

Following those rules will prevent any further fish death due to water change.

Sujit Modak

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