Does your fish die after changing the water? Here’s everything you should know

Does your fish die after changing the water Here’s everything you should know

One of the prerequisites of fish keeping is changing the water of the tank after regular intervals. It is one of the facts that every fish owner knows. However, everything goes smooth while doing so is not necessary. Many aquarists often see a dead fish in their tank after a few hours of changing the water, especially the beginners.

Fishkeeping is one of those hobbies that come with a huge responsibility. Although keeping any pet is a responsibility, when you have a fish, things may get a little complicated. The reason behind this is while other pets live closer to your proximity, you can understand their behavioral changes. However, in the case of fish that live in their closed water ecosystem, watching them all the time to examine their behavioural pattern gets difficult.

You fed your fish right, checked all the water parameters to be proper and changed the water just a few hours before. Everything was going well. But, alas! When you check the tank in the morning, the poor fish is dead. You see it floating upside down. It is a real heartbreak, and you wonder what when wrong.

The fish, which was hale and hearty a day before and showing all the signs of being healthy, suddenly dies. It may come to mind does the water change kill it? Apparently, the answer is yes, but it is more explanatory.

There is a tale of an aquarist who lost all his fish after changing the water in the tank. Everything has been going fine in his aquarium since the day he has bought it. All the fish were healthy, the plants were doing fine, just that the tank had started to look dirty over a year.

One day a friend visited him and saw his beautiful fish and plants. But seeing the dirt and scarps on the sides of the tank, the friend suggested him to change the water and clean the substrate. At this time, the aquarist also came to know that he must regularly change the tank’s water. Without wasting time, the aquarist transferred his entire stock into a big bucket, changed all the water, cleaned the substrate, removed the scarps from the sides and put the fish back in the tank.

It was so thrilling and satisfying for the aquarist to see his aquarium immaculate and fresh all over again. When he checked his aquarium the next morning, half of the fish were floating upside down. It was so devastating to see them dead. Within a week, the rest of the fish died too. The aquarist and his advisor friend blamed the tap water for this loss.

Was it only the tap water to be blamed? Have you ever experienced such an incident? Let’s dig a little deeper and know the possible reasons for the fish deaths.

Fish death due to water change 

As we all know, partial water change is necessary to keep the water clean and healthy in a tank. However, it needs to be done only under proper guidance and after having detailed knowledge about it. If the fish is dying immediately or after a day of water change, the water chemistry and temperature can be blamed. A lot of water parameters alter once you change the water, like water pH, hardness, temperature, micro and macronutrients, etc. An abrupt change may cause the fish to die immediately.

So, the possible reasons could be-

  • A drastic change in water parameters
  • Sudden change in water temperature
  • Elimination of beneficial bacteria

Change in water parameters

Water chemistry keeps on changing inside a closed water ecosystem like an aquarium. Fish excreta, leftover food particles and dead plant matters change the water chemistry. But since the change due to these factors is slow, the fish keep on accustoming and coping with the changes.

However, when this change is abrupt and drastic, fish come under tremendous stress. Sometimes it is not possible for the small creatures to adjust their little bodies according to the new water variables that make them sick, eventually leading to death. Changing aquarium’s water without taking necessary measures and replacing everything is more damaging to the fish compared to not changing the water for a long time.

Even though you replace the old water with the fresh and purest one, the fish may not attune to this newness and drastic changes in pH, hardness and other water parameters. Some fish may die immediately, and some may fall sick, leading to their death over a week or so.

Change in water temperature 

Another potential reason could be a sudden change in water temperature. When there is a vast difference in temperature, fish come under shock. In such scenarios, some fish die within a few minutes, and others fall sick and die over the next few days.

Elimination of beneficial bacteria 

When you change the tank’s water inappropriately, it acts as a slow killer to the habitants. Over time the aquarium develops beneficial bacteria that help to establish a nitrogen cycle in the tank. It prevents the toxic elements from accumulating. So when you replace the water entirely and clean the tank thoroughly, a majority of beneficial bacteria gets eliminated, ultimately deteriorating the water quality. It makes the fish weak and sick.

There is a number of pathogens floating in the water along with the fish. When there are beneficial bacteria, these pathogens stay within their limit and do not pose any threat to the fish. However, as soon as the beneficial bacteria get low, the pathogens do not leave the chance to attack and succumb the fish to death.

Then why should I change the tank’s water? 

When there are so many cons to changing the tank’s water, then why should I do it in the first place? It is one of the most frequent questions that new aquarists ask.

Well, changing the water is one of the prerequisites of having an aquarium, and there are plenty of reasons behind it.

  • Eliminates the dissolved toxicity– There are plenty of biochemical processes happening inside a tank that increases the toxic chemicals in the water. Dissolved impurities are not visible to the naked eyes. If left as such, the toxic chemicals keep on accumulating, which could be fatal for the fish. It is one of the reasons why a partial change of water in regular intervals is necessary. It restricts the buildup of toxins and keeps the water suitable for fish’s health.


  • Removes the leftover food and fish excreta– When a fish eats, it will excrete too. Now all this fish excreta accumulates at the bottom of the tank. Likewise, the leftover food of the fish also goes to the bottom and mount up on the substrate. If these accumulated products are not removed on time, they start breaking down and decomposing into ammonia which is a highly toxic gas.

If the nitrogen cycle is established in the tank, ammonia will convert into nitrate, which is less harmful. Nitrate will not kill the fish immediately but will certainly reduce the fish’s immunity. Therefore, it needs to be eliminated from the tank as well, for which a nitrogen cycle is necessary. Also, aquarium plants help in getting rid of harmful nitrates.


  • Oxygenates the water– Oxygen-rich aquarium water is healthy water for fish. We all require oxygen for survival. Even though fish can’t breathe, they absorb the dissolved oxygen through gills. A regular water change ensures that the tank’s water is well oxygenated and is suitable for fish’s survival.

The quality of the tank’s water directly impacts the health and immunity of fish. A fish that lives in well-oxygenated and toxins free water will surely have stronger immunity. It will rarely get sick.

What is the right way of changing the tank’s water? 

Sudden, abrupt and complete water change is one of the main reasons for fish’s death in an aquarium as they come under shock and stress. However, if everything is done slowly and fish are given enough time to acclimatize, they will not come under shock. Instead, adjust easily. A few things that you should keep in mind while changing the tank’s water are-

Change the water partially– Replacing a small amount of water at regular intervals is far better than a massive water change once in a while. It is advised to change only 10 to 15% of the water weekly and one-third monthly to keep the fish sanity.

Use a vacuum cleaner to clean substrate– When you are performing the major changes in the water, suck out the debris from the bottom of the tank with the help of a vacuum. Cleaning the substrate once a month is enough to maintain the water quality. Invest in a high-quality vacuum for siphoning out the debris.

Check out the water parameters– Once you are done with changing the water, analyze the different water parameters like pH, temperature, hardness by performing a simple water test. Invest in a testing kit each time you perform a complete water change and ensure that everything is within the range. Make sure that the temperature of changed water is close to the aquarium’s precious water.

Note: If you are adding tap water, do not forget to treat it with a conditioner. Also, make sure that the water is free from heavy metals and chlorine.

No matter how and when you are changing the tank’s water, acclimatization of fish is what you need to keep in mind. Never impose your fish on a sudden change. The chance of inflicting your precious fish to risk is small when you practice partial water change, but if you never change the water, there are greater chances of fish getting sick due to increased dissolved toxicity and decreased oxygen.

Final words 

We understand that beginner aquarists are often in apprehension when it comes to changing the tank’s water. Plenty of questions occupy the mind. We hope that this article has provided some insight into the importance of regular water changes and made you understand that it is absolutely an essential part of fish keeping.

All you need is to perform water change in a way that fish adapts easily. Sudden change brings disturbance in the fish’s system, which is the main cause of their death. But the good part is you can avoid fish death due to water change by keeping the above points in mind.

If you are performing the water change on a regular basis, the fish also get habituated with it. It is when it becomes easy for them to cope with the water change and adjust their body at the same time. When the water quality is good, the chances of fish getting sick reduces, helping them live healthier and longer.

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