The Reasons Your Fish Are Dying – And How To Fix It

It happens to all of us, even for me! Your fish dies sometimes. It could even be that your whole fish tank dies and that is heartbreaking!

So, why are your fish dying? There are several reasons for the fish to die. The most common is due to stress but it could also be of disease, age or by a mistake from you as an owner.

No matter the reason there’s an explanation. I will go through what to look for and how to know what the reason is and how to prevent it.

Death by stress

Stress is the most common reason for a fish to die. It happens for most fish keepers and some things that might stress a fish are:

  • Transportation
  • Change in environment
  • Stress by other fish
  • Disease

Stress is connected to some kind of problem. The stress is slowly killing the fish and all of a sudden it is dead. That makes it hard to determine if it’s the stress or something else killing your fish.

Some fish are more sensitive to stress than others. One of my favorite fish is Otocinclus. They are unfortunately very sensitive to stress and you should care for them with extra caution to not stress them.

To prevent stress you should get cautious in the store already. You have read and chosen which fish you want and when coming to the store and look at the fish you see some hanging in the back of the tank and not swimming lively.

These are the fish you don’t want to take home. Those are signs of something is wrong and chances are they will get stressed when moved to a new tank.

Be careful when transporting your new fish home from the store. This is the next risky part after choosing fish. Transportation is a stressful process for every fish. Put the fish bag in a dark box that is able to keep the temperature well. If the temperature fluctuates during transport the fish will get more stressed.

Keep the box dark and don’t peek during the transport. Try, if possible, to make the way home as smooth as possible.

Now you are finally home. Time to put in the fish in your tank! But don’t just put them in!

You need to acclimatize the fish for their new environment. Your fish tank and the fish tank they were stored in does have different parameters. That change in environment could stress and ultimately kill the fish.

Now you have put in your new fish as you should but the dangers are not over yet! Other fish that already live in the fish tank could be very mean to their new roommate. The fishes already living in the tank could bully the new poor fish which leads to death by stress. More on that later.

Even if everything is fine this far the danger of stress is not over yet. The biggest dangers are over but there are always dangers that could stress a fish to death.

It’s the danger of disease. There is nothing else to do than to keep the water parameters good to prevent an outbreak of a disease. A lot of diseases will kill the fish but if a fish gets sick, you put in medicine and the fish get healthy, the fish might still die due to the stress from the disease.

Death by old age

Like everyone and everything, even fish ages and they are not able to live forever.

If you have had a fish tank and your fish for a long time and never had any problems with anything (more on that later) and your dead fish hasn’t shown any symptoms of being diseased before it took its last swim, then you could probably say it’s due to old age.

If you bought the fish from someone else, that person could have the fish for a long time. That’s why it’s good to ask if you buy or get fish form someone else to ask them how long they have been keeping the fish.

It would be very sad to get a new fish that’s old and it dies a short while after you got it.

How long a fish lives depends on the breed. Some fish only live for a year while others could live a lot longer. It’s something you have to read about before getting fish. When getting a new fish you should read about that breed to know what that particular species needs and what you could expect from that fish.

How long they live and how sensitive they are for diseases etcetera, is a crucial part to know for not getting your heart broken right after getting a new fish for your fish tank.

Now, the cause of death is not only due to old age. There are plenty of other reasons for a fish to die.

Death by disease

One thing to really be aware of is the risk of diseases. Your fish could be ill, just as you can be. The diseases are deadly to all fishes. You should be ready with some medication in your cabinet (medication for fish there is) if any of your fish are getting sick. I have some recommendations on which medication is the best at my recommendation page.

If you are looking after early signs of disease you have a better chance of not threatening the life of your fish by letting them get diseases. If you medicate early you will have a better chance of stopping the disease. If your fish tank will get a break out of disease it will threaten the whole tank and you are running the risk of your whole tank to be wiped out.

Every day when you look at your fish you should look out if something seems unusual with your fish. For example, the fish is breathing more heavily and swimming strangely. If it’s the case you should put in the proper medication right away!

Some of the most common diseases are ick, dropsy and fin rot.

Ick is white spots appearing at the fish. The fish will also scratch itself against the furniture and stones in the fish tank. As soon as you see the signs of it, put in proper medication to heal the fish before it’s too late!

Dropsy is making the fish swell up and the scales are not slimmed towards the body. If it’s in that state it’s almost impossible to save the fish but try to put in medication. Earlier signs are that the fish is not eating. This is hard to detect in an early state.

Fin rot is a disease, as the name suggests, where the fins seem to rot. As soon as the fins look different you should put in medication.

There are a lot of different diseases but the most important part is to spot that something is different when you are doing the daily check-up on your fish. In that way, you are able to stop a potential disease that could kill your fish.

Keeping good water parameters is a good way to prevent diseases too.

Bullied to death

A fish could be bullied to death. It’s most common that a new fish is getting bullied to death when coming to a new home where there are other fish already living.

The fish already living in the fish tank might see their new friend a threat and might attack it. Most of the time the newcomer dies.

It’s also good to know the temper of the specific species of your fish. Some species are more aggressive than others. The more aggressive fish should normally be kept by themselves due to their aggressiveness.

The betta fish is a clear example of a fish being aggressive to others. They should be kept by themselves because they might attack others and kill them. That’s something you should research before getting a fish.

Death by bad water parameters

One thing that is lethal to all of your fish and are, most of the time, easy to prevent is bad water parameters.

Ammonia, nitrite, and nitrate are the most lethal water parameters that could kill all of your fish in an instant. Other parameters that are lethal to your fish if not kept under control is pH and temperature.

To find any early signs of an imbalance in your water parameters you should do water tests regularly. Which tests to use have I written about in my recommended tests page.

The absolute crucial test to make is one of nitrite. Nitrite will be broken down to nitrate. Nitrate is something your fish will tolerate but is lethal in high doses. Nitrate is made when nitrite is broken down by the good bacteria in your fish tank.

Nitrite though is much more lethal to the fish and could kill the fish in lower doses than nitrate does. It is a good idea to keep an eye out for an increase in nitrite to be able to see early signs of a lethal spike.

Ammonia is even more lethal than nitrite but it’s also easier for the bacteria to break down into nitrite.

To prevent such spike you need to be doing your water changes and make sure the bacteria in the filter are doing well. Give the bacteria a bacteria starter every month and do weekly water changes of about 15-20% to keep control over your ammonia, nitrite and nitrate levels.

If the temperature is too high the fish will die. What’s too high is depending on the fish you have. Different fish need different temperatures. It’s important to keep fish within the same temperature range. What’s good for one species could be bad for another and that could kill it.

Also, watch out for temperature changes in the room around as it will affect the water temperature in the tank too.

pH is also something that differs depending on the fish you keep. Some need high pH and others need low pH. Search what pH the fish you are interested in needs and keep the fish in the same tank within the same pH-range.

pH is controlled by measure it with a water test. If the pH is too low you need to raise it. You could do it by putting coral sand in or crushed shells.

If the pH is high and needs to be lowered on another hand you could put peat into your filter and the pH will drop. Be sure to buy it from the store and not picking it from outside.

Picking peat from outside might bring in infections into your fish tank and it’s harder to control since you don’t exactly know what’s in the peat.

Death by bad routine

The last thing that could kill your fish is that you having a bad routine with your fish.

That might be not doing water changes as often as you should, or at all. You might skip doing water tests because the water looks clear. You might skip giving the fish food or even too much food.

All these things are something you need to have a routine on. Otherwise, you will miss early signs of possible fatal conditions for your fish.

Write up in your calendar what to do. It doesn’t take much time to do the necessary things for a fish tank. Let’s not forget we are handling living animals and they need our time to be well.

Take a look at all your fish for something unusual and count them when you wake up. Give a small amount of food and continue your day. In the evening, do the same procedure. Do water change once a week and a bigger with added bacteria once a month.

It’s not more than that you need to do. And by having a routine you eliminate a lot of causes of stress for your fish and your fish will live happier and longer for you to enjoy.


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

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