Why is my fish dead

Help my fish die!

If several aquarium fish suddenly die within a very short time, then every aquarist's nightmare comes true. Here you can find out what the causes of the drama in the aquarium can be and how you can save the remaining fish.

Possible causes of fish death

There are a few typical causes that are quickly verified. It is important that you have some accessories to check the temperature and water values. It doesn't cost much and is worth its weight in gold in such situations.

Too high temperature

A common cause of fish death is too high a temperature. Often the fish swim around apathetically, lie on the ground or gasp for air on the surface of the water. Check your aquarium heater and measure the temperature with an aquarium thermometer. The aquarium heater can heat up too much for various reasons, maybe it was set incorrectly or the thermostat is broken.

The solution: set the heater back on correctly or replace the heater in the event of a malfunction. Furthermore, a small partial water change with slightly cooler water cannot do any harm.

Too much CO2 in the water

If you use a CO2 system, it can happen that the CO2 concentration is too high. As a result, your fish have too little oxygen available to breathe. This mainly happens at night because the plants do not use any CO2 or produce any oxygen during this time. Measure the CO2 levels in the water to see if that's the reason.

The solution: Carry out a small partial water change and, if possible, set the filter outlet so that the water surface is well agitated. This promotes the exchange of CO2 and oxygen.

Nitrite / nitrate concentration too high

If you don't perform partial water changes regularly, the water quality can deteriorate. Too much food or too much fish in the aquarium also lead to an increase in pollutants in the water. A high nitrite value is a problem, detectable nitrite is poisonous! Do a nitrite and nitrate test with your water test kit.

The solution: Change about 1/3 of the water as quickly as possible. To avoid such situations, change the water regularly every week (at most every 2 weeks) in the future.


Diseases can spread quickly in the aquarium and infect all fish. It is therefore very important to identify and treat diseases quickly. You can recognize disease by looking closely at the dead and living fish and checking for changes. You can find typical symptoms of diseases on our page about ornamental fish diseases or in web forums.

The solution: Find out if your fish have a disease and which one it is. Get the right medication from the pet shop as soon as possible and treat your fish.

Predatory fish

The solution: If you do have a predatory fish in the aquarium, you either need to remove the predator or the species of fish it is attacking. As a first step, catch the predatory fish and place it in a bucket or another tank if available. Then you can take care of where you put the fish. You can either give them to well-known aquarists or try to return them to the pet store. In any case, call beforehand and clarify whether a return is possible. Another option is to swap the aquarium for a bigger one, but that doesn't work overnight. Find out whether the fish species you keep together can generally be socialized with one another.

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