Why Do Fish Die After Changing The Water

Fish make excellent first pets for kids. They teach them responsibility and give them something to call their own. For many reasons fish can pass away. Then there is the dreaded flushing of the fish and trying to explain death to your child. Then the debate on whether or not to purchase another fish.

The main reason is because it is too big of a shock to their system. This can happen because there is too big of a difference in water temperature. It can also occur if chemical or pH levels are too high/low.

We will take an in-depth look at reasons why your pet fish dies when you change the water. We will give you options and suggestions on how to prevent this occurrence. We are also going to do the best that we can to educate you and help you prevent the fish funeral.

What Has Gone Wrong

Fish dying after a water change can be for a simple reason or it can be more complex. We often never know why our fish die, we just speculate on what may have been the cause.  

Fish can die because of too much fish waste, uneaten food particles, or dead leaves from your live plants. These can cause infections and disease if your filter is not working properly. Your filter is made to protect your fish, and help to keep the tank clean and oxygenated.

If there is a large water change it may cause a drastic shift in the makeup of the tank. This is why it is recommended that you only perform a 20-25% water change every week. You only have to do a full tank change and scrub every 3-4 months. 

If your fish doesn’t die right away, it will suffer from stress from the massive change. Stress leads to not eating, no resting, and possibly getting an infection. It may take a fish a couple of weeks to pass after a water change. They will do their best to fight off and reduce their stress, but it doesn’t always work.

Should The Water Actually Be Changed

If there is a potential that your fish will die, then why change the water? Water changes ultimately lengthen the lifespan of your fish. Contradictory, I know. But for long-term health of aquarium fish, you should change 20-25% of the water every week.

Fish are exposed to all kinds of bacteria, parasites, and viruses more often than we realize. If you have fish with very strong immune systems then you have nothing to worry about. They will recover very quickly, if they get sick at all. But if you have a fish that is unhealthy to begin with or does not have a very strong immune system, then it is only a matter of time.

Your fish that do not fall prey to infections or disease can pass for other reasons. Elevated nitrates can be a problem for some fish. Poor water conditions, in general, can impact your fish. Especially if you have young fish that are just beginning to grow. If your filter is not working, that can really impact your fish.

What Should I Do After A Fish Dies

You need to realize that you need to be very observant if you are going to have an aquarium of fish. Whether you have 5 fish or 20 fish, there are things that you need to do and check every single day.

You will need to keep an eye on the temperature, pH, and other levels, and your fish in general. Count them each morning when you feed them. Observe if they are acting “normal” and active. Are they all interested in the food? Eating like they normally do? 

If you are missing a fish, you need to check everywhere in the tank and quickly. Check the corners of the tank and remove the lid so you are able to look down into the water. If you are still not able to locate the wayward fish then you need to look deeper. The fish could be slightly under the rocks or other decor. It could be stuck to the filter inlet or lying on the bottom of the tank (and you overlooked it). Be very thorough in your search, in case it has in fact died.

If you find a fish has died, you need to remove it immediately. The sooner after it dies, the better. The dead fish will pollute your tank water and quickly rot depending on how warm the water temperature is. If the fish happened to die of an infection or disease you do not want the other fish to start picking at the corps. If they do, it may not take very long before your whole tank is infected. 

Once you have removed the dead fish you need to immediately test the water. You will need to be prepared to fix the water situation right away, if the need arises. The quickest way to do this is with a syphon and a bucket. You will also need a thermometer, new filter, bacteria booster and your test kit. You will need to test for nitrite, nitrate, pH balance, and ammonia. Make sure you know exactly what a safe temperature for your fish is. Every species is a little different. 

Be wary, some fish (i.e. catfish) tend to wedge themselves into very tight spaces, like rocks or other decor. Playing possum. This makes them very difficult to find and when you do, you do not know if they are trapped or dead.

Other Things To Do When A Fish Dies

If you are unsure what the cause of death was, it doesn’t hurt to seek out advice. Testing your tank water is going to go one of two ways. It is either going to say that all levels are fineor it will give you every indication that there was a problem with one (or more) of the levels.

Make sure to also check the temperature of the water, because a drastic change can be fatal for a fish. If your heater stopped working and the water got too cold, that would cause death in a fish. 

If your tests all show that your water quality is fine, then you need to look at other reasons for your loss. Some of those reasons/causes could be; starvation, long term stress, disease, over-feeding, or bullying. Take a good look at your tank, evaluate what is lying in the bottom (the amount of food). Look back on how often you feed your fish.

Look at some of the other species and see if there are tiny pieces of their fins missing. If there are, this could indicate you have another fish bullying in the tank.

Check out all of the other fish of the same species. So they seem to be acting ok? Are they eating, swimming, and playing? If so, great! If not, keep a close eye on them as you could lose them any day. It could be for multiple reasons, most are listed above. But it may also be there was something wrong with that batch of fish when you initially purchased them. 

Find a credible person at a pet store or exclusive fish store. Take all of your information in. Make sure you have all of the test results, your tank size, the number of fish in the tank, the different species of fish, filter model, and food that you feed them. If they are knowledgeable about aquarium fish they should be able to offer you some credible reasons for the death. 

Why Do My Fish Die and How Do I Prevent It From Happening Again

It is very rare for a fish tank to suffer mass deaths. One or 2 is pretty normal, but if you have fish after fish after fish pass away, then there is something definitely wrong somewhere. There are many reasons why fish die, including the following;

  1. New tank syndrome: we have covered this in a couple of our other articles. Before your tank has reached safe levels of chemistry, there are high levels of ammonia and nitrates in the water. These can be very fatal to your fish. It is actually recommended you wait 3-6 weeks for your tank to ‘cycle’ before you add your fish to the new tank set up. This gives your levels time to reach the safe zone for your fish and it also gives fresh, healthy bacteria time to grow. 
  1. Rapid water changes: when you have a well established healthy tank, everything is in balance. Bacteria, plants, and fish. When you suddenly change a very large quantity of the tank water it disrupts that balance. It shocks the fish, throws the chemistry out of whack, and the fish can die. Water does need to be changed on occasion. It is recommended that you change 20-25% of your tank water every week. *TIP: leave the new water on the counter for 48-72 hours before the change. This gives the water time to reach a temperature that very closely matches the tank.*
  1. Water quality: as stated earlier, overall water quality is essential. pH levels, filtration system, and NOT over-feeding are a big part of keeping the water healthy. If you overfeed you have excess food that can plug the filter. If your filter isn’t working properly the oxygenation in the tank is affected and your fish will start getting sick. Make sure you check your pH, nitrates, nitrites, and oxygen levels weekly. Especially after a water change or removal of a dead fish. *before you purchase your fish, learn the perfect chemical balance and temperature levels for that species of fish. This will give you a leg up on preventing death.
  1. Temperature changes: a sudden spike or drop in temperature can drastically impact your fish. Most aquarium fish can adapt and tolerate a certain range of temperatures. But when too drastic of a change in either direction can cause stress on your fish. Make sure you check your tank heater, at least, every other day. If you keep your house at a consistent temperature 365 days a year, you may not need the tank heater.
  1. Unexpected toxins: you need to protect your fish and the water by making sure your tank lid fits properly. This will keep any harmful chemicals out of your tank water. Bug spray, hair spray, cleaning chemicals, soaps, lotions, and more can be very hazardous to your fish. Make sure that you wash your hands with a non scented, non antibacterial soap before putting your hands into the tank for any reason at all. If you forget, or you feel that something may have gotten into the water that can harm the fish, immediately check the levels and go from there. 


Stress is the #1 reason that fish die. It is not good for human health and it isn’t good for animal health. It may take a few days or it may be a few weeks for your fish to die due to the stress

Unfortunately, there are many causes of stress for fish. Some we covered in this article. Because there are so many stressors for fish, you may never know the exact reason for their passing. Your aquarium could be too small, your fish is old, it has a disease, it was unhealthy when you purchased it, or any number of other things.

Routine maintenance; 20-25% water change every week, filter change every month, testing pH and other levels regularly, and making sure fish get enough rest are the best ways to prevent death in your fish. 

Fish are spectacular creatures to observe, and as I stated in the beginning, the perfect first pet. But please make sure you do your research before you purchase your fish (while your tank is cycling, for 3-6 weeks). Make sure you know all the ins and outs of the species you are wanting in your aquarium. From the food to the temperature, to the care of the fish. 

Make sure you understand how to do all of the routine maintenance that is involved with having an aquarium. The water changes, the chemical balances, and everything else involved. As long as you know what you need to and are confident in taking care of your fish, then they should live a long and happy life. 

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I'm passionate about fish pets and love sharing everything I learn about them.

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