Why did my fish die immediately after the water change?

In this article, we will answer the question “Why did my fish die immediately after the water change?”. We will discuss the death causes and how to avoid them.

Why did my fish die immediately after the water change?

Fish can adapt to changes in their environment throughout time. Changing a big percentage of aquarium water, on the other hand, may have a major impact on the characteristics of the remaining water in the tank. A abrupt shift in these parameters may cause stress or death in the fish, depending on how well they can acclimatize to the new conditions. This could happen even with a minor water change if it has been a long time since you last did it.

What are the causes of death and what can be done to avoid them?

During a water change, a variety of things could go wrong. Here are some of the water conditions that could be causing your fish to die: 

1. Before you changed the water, the nitrate levels were excessively high

A high concentration of nitrate in aquarium water can be toxic to fish and could cause them to perish. However, a fast drop in Nitrate levels after a water change could cause Osmotic Shock in your fish. 

Nitrates are ions (NO3-), and fish can adjust to variations in the quantity of ions and ionic compounds in their environment, such as changes in the amount of salt in their environment, throughout the course of a lifetime. Fish that live in freshwater have higher levels of ions and ionic compounds in their bodies than fish that live in saltwater.

This implies that if the fish’s body fluids aren’t regulated, the surrounding water will try to get into the fish’s body, drawing the ions out through a phenomenon known as osmotic pressure. The osmotic pressure that affects saltwater fish acts in both directions and can change directions at any moment. 

Water regulates the saltwater fish’s body because it contains more ions than the fish’s own body, which keeps the fish’s internal organs from taking too many salts from the water (salts are ionic compounds). Osmotic shock is a condition in which a fish’s physiology is unable to regulate its absorption of ionic substances and, as a result, the fish either absorbs or discharges an excessive amount of water.

Fish may suffer from osmotic shock when a considerable amount of the Nitrate ions in their tank is removed by changing the water. Freshwater fish that are subjected to osmotic shock following a water change can retain an excessive amount of moisture, which can result in organ growth, a disease known as Dropsy. The enlarged organs may exert pressure on the fish’s swim bladder, causing it to swim upside-down or sideways. In truth, a bloated Betta fish is frequently caused by a problem with osmoregulation. 

Impaired osmoregulation could also be the cause of your fish’s irregular swimming and agitated behaviour following a water change. This strange behaviour may be followed by spasms. A liquid water test kit could be used to determine whether the rapid change in Nitrate stressed your fish. Test strips are imprecise; nevertheless, a liquid test kit will allow you to study the qualities of the water in the most exact manner possible. 

It is recommended that you change no more than 10% of the water in your tank to prevent stressing or killing your fish due to Osmotic shock. Wait around an hour or so between partial water changes if you need to exchange a bigger amount of water.

2. The new water’s temperature was too cold for the tank

If the temperature of the new water does not perfectly match the temperature of the water in the aquarium after a water change, your fish may develop ill and die. The majority of fish that become unwell seemingly out of nowhere are due to water temperature variations. Fish do not regulate their body temperature because they are cold-blooded organisms. When you replace the water in your aquarium with colder water, the fish experience thermal shock, making them extremely susceptible to disease. 

Thermally stressed fish do not move much, lose their colour quickly, and may die practically soon following a water change. As a result, the immune systems of the fish that survive could be harmed. They may now be vulnerable to ailments that their systems could previously fend off while they were well. 

Extensive parasitic Ich outbreaks, which can be difficult to cure in freshwater fish and more more difficult to treat in saltwater fish, are usually triggered by changes in water temperature. To prevent fish deaths due to temperature shock, perform water changes using water that has been preheated. 

3. You neglected or inserted a dechlorinator too late 

You’re asking for trouble if you don’t use a dechlorinator before adding tap water to your fish tank. Chlorine or chloramine can be found in large quantities in tap water. Both are toxic to fish and will kill nearly anything in your aquarium. If you want to prevent harming your fish, use a good dechlorinator like Seachem Prime (which is the dechlorinator of choice for the majority of fish keepers) while doing water changes.

In any case, when some aquarists replace the water in their aquarium, they first add the new water before adding the dechlorinator. Because even the greatest dechlorinator takes time to neutralize Chlorine and Chloramine, they risk poisoning their fish this way. 

Chlorine is normally neutralized in less than a minute if you follow the dechlorinator’s directions. Most water treatment facilities, on the other hand, choose Chloramine for water disinfection since it has a stronger chemical bond and evaporates more slowly. For a given volume of water, it might take up to 5 minutes for a good dechlorinator to completely neutralise all of the Chloramine present. Adding additional water before the dechlorinator may not be a big deal in larger tanks. 

However, in smaller tanks, the effects of the alteration could be fatal to your pet fish. Also, chlorine and chloramine are bacteria killers by definition, something I don’t believe many people are aware of when it comes to disinfection. When you replace the water in your aquarium, don’t use chlorinated water since it will kill most of the beneficial bacteria that are responsible for the system’s ongoing nitrogen cycle. 

These bacteria convert poisonous chemicals like ammonia and nitrite into less hazardous nitrate. An ammonia or nitrite surge is a distinct possibility following the replacement of the water in the fish tank with chlorinated water, which indicates that the water has been chlorinated. Fish gills are harmed by ammonia and nitrite poisoning, which causes respiratory distress. 

This might explain why your fish appear to be dying and gasping at the top or bottom of the tank quickly after a water change. Water should be dechlorinated prior to being added to the tank in order to avoid damaging the biofilter or harming your fish with chlorine water.

4. You changed the filter medium and/or vacuumed the entire substrate

Porous surfaces are home to the beneficial microorganisms that reside in aquariums. Your filter medium and substrate are both porous surfaces that provide a great breeding environment for these bacteria. 

When you vacuum the whole substrate at once during a water change, you run the danger of losing a significant amount of the bacteria population. Because of this, it is possible that the nitrogen cycle in the fish tank will be impeded. Changing all of the filter media is considerably riskier because it contains a lot of helpful microorganisms. 

Surprisingly, changing filter media while cleaning the tank might cause ammonia or nitrite levels to rise, quickly killing all of your fish. This is a good indication that the Nitrogen Cycle has resumed in your aquarium if the water in your aquarium gets murky white a few hours after you completed the last water change or cleaning session. 

After cleaning your fish tank, rinse the filter media with dechlorinated aquarium water and vacuum no more than 30 percent of the substrate every session to prevent injuring your fish.

5. Your tap water included an excessive amount of saturated gas 

When there is too much-saturated gas in the water, it wants to escape as soon as possible, bringing the pressure between the liquid and the atmosphere back to normal. After a water shift, this impact could be deadly to fish because gas bubbles could accumulate in their blood, fins, and eyes. 

If bubbles form in the fish’s bloodstream, it may die, or if bubbles form in the fish’s eye, it may lose its vision. A liquid’s gas saturation is determined by its temperature. If you quickly heat water, you’ll notice little bubbles emerging inside its container as a result of the sudden temperature shift. This is due to the fact that warm water contains less gas than cold water.

Simply swirl the fresh batch of water thoroughly before making a water change to avoid creating Gas Bubble Trauma to your fish. Another efficient method of releasing saturated gases is to spray water from above the aquarium’s water surface, resulting in splashing. The best strategies for stimulating gas exchange between water and ambient air include splashing and surface agitation.

What is the best way to replace the water in your aquarium without harming the fish?

To avoid fish fatalities following a water change, you must follow a few basic measures. One of the most essential things to remember in this situation is to make adequate preparations before adding more water to the tank. Before you begin, you’ll require several low-cost yet important items: 

–       A bucket or a container to hold the water

–       A little aquarium heater 

–       A water dechlorinator 

Now that you’ve gotten everything out of the way, you may change the water in your aquarium safely. 

To do a water change without risking the death of your fish, follow these exact steps: 

1. Fill a container with the required amount of water. I normally use stainless steel or plastic bucket. Make sure the container you’re using doesn’t contain any copper, as copper is hazardous to fish and invertebrates. 

2. The fresh water should be heated until it reaches the same temperature as the tank. In order to heat the water in the container, a small, adjustable heater is used. If your main tank has a preset heater, you might use the same brand to heat the new batch of water. 

3. Wait for 5 to 10 minutes after dechlorinating the water. This is done to give the dechlorinator enough time to fully eliminate the harmful Chloramine. 

4. Stir the water in the container for approximately one minute. In this approach, you are contributing in the gaseous exchange between the water and the surrounding environment.

5. During each session, change no more than 10% of the aquarium water. If you need to replace more water, wait an hour or two to allow your fish to acclimate safely before adding the second batch. Partially changing the water will likely prevent the fish from experiencing any shock or stress.


In this article, we answered the question “Why did my fish die immediately after the water change?”. We discussed the death causes and how to prevent them.

If you have any questions or concerns, please let us know in the comments section below!

Frequently Asked Questions (FAQs): Why did my fish die immediately after the water changes?

Why did my fish go so quickly? 

Rapid Changes in Water: Water chemistry is meticulously balanced in a healthy, established tank with resident fish, plants, and microbes. Changing significant amounts of water quickly will disturb the chemistry and shock the fish, resulting in death. Extremely rapid changes can be fatal. 

Is it necessary to feed fish after a water change? 

There’s no law about feeding new fish right after they’ve been acclimated, but the fish will let you know. Some fish will immediately go swimming, while others will hide. This article will provide you with some wonderful advice on how to feed your fish once they’ve joined your family. 

How can I tell if my fish is suffering from ammonia poisoning? 

The fish may appear to be panting for oxygen at the surface. Their gills will change color to a reddish-purple hue, giving the appearance that they are bleeding. Your fish’s hunger will diminish as their biological systems degrade, and they will grow increasingly sluggish as a result. 

What is the most effective technique to keep track of the oxygen level in my fish aquarium? 

An aquarium water dissolved oxygen metre, which can be carried about with you, is the most accurate way to monitor the level of dissolved oxygen in your aquarium water. After calibrating the metre, insert a probe into the tank water and watch the readings appear on the digital display of the metre.

What’s the deal with my fish panting for air? 

The scarcity of dissolved oxygen in the water is the reason they are seeking to breathe at the surface. Insufficient aeration or poor water quality can result in low oxygen levels, which can cause stress in the fish. Depending on the season, fish require varying amounts of oxygen. 

What is the maximum amount of time a fish can survive without water? 

Fish can survive for around 10 minutes outside of the water on average, but they can expire much more quickly if they land on a surface that absorbs their moisture. Saltwater fish, on the other hand, tend to live longer, and certain fish, such as amphibious fish, have special adaptations that allow them to survive outside of the water for extended periods.


Why Do Fish Die or Get Stressed After a Water Change? 2020. https://aquanswers.com/fish-died-after-water-change/

Sharpe, S. 2022. Can Massive Water Changes Kill Fish? https://www.thesprucepets.com/can-massive-water-changes-kill-fish-1381885

Barrington, K. 2021. Can a Routine Water Change Kill Your Fish? https://www.ratemyfishtank.com/blog/can-a-routine-water-change-kill-your-fish

Sardar, S. Why Fish Die after a Water Change and How to Prevent It. https://creature-companions.in/why-fish-die-after-a-water-change-and-how-to-prevent-it/