Causes and signs of stress in fish

The constant progress of ichthyological studies proves that fish are much more intelligent creatures than we think, so they are able to recognize each other and remember social relationships, in this sense, they can also suffer and feel stress, as cats and dogs would. Therefore, in this article we will show you some of the signs that may indicate that our fish is stressed, but we will also review what could be the causes of this problem. 

As with humans, continuous stress in fish could have serious and harmful consequences on their health, however, the causes could be simpler than in complex organisms, since they are usually related to the environmental conditions that exist in the fish. the environment of the fish. 

In this sense, many people do not know that maintenance could be more complicated than thought, for example, the filtering system must be related to the characteristics of the fish tank, in addition, the temperature and chemicals in the water may not be compatible with fish, among many other inconveniences that we can find. For these reasons, below are the most common causes of stress in fish.

5 common causes of stress in fish

1. Chemicals in the water

Some chemical compounds derived from nitrogen could be toxic to a greater or lesser extent for the fish. Such is the case of ammonia, which is formed naturally by fish waste and food waste, but in all circumstances is a contaminant for him and could cause high levels of stress. For this reason, an optimal fish tank must mainly have a filtering system, to quickly dilute and transform the different contaminating substances that are in the water.

2. Water conditions

The temperature must be according to the type of fish, since maintaining a very low or high temperature is another probable cause of stress. A common example is goldfish, which can live peacefully in temperatures as low as 20°C, however, they can also survive the winter in freezing temperatures, but these conditions eventually cause them a lot of stress.

It is important to know that not all fish come from the same type of water, so introducing them to an artificial habitat that is very different from the natural one can be deadly. Although certain species can survive in various types of water, the vast majority are specifically hard, soft, fresh or salt water. For example, those that are intolerant to salt usually have no scales or some very small ones.

3. Fish tank size

Small or very large fish tanks create conflicts and stress in the fish, so when purchasing the best fish tank it is important to take into account the space necessary for the fish to swim freely. You will find tanks from 40 liters that the fish can occupy properly, but some species need up to 400 liters to feel comfortable.

In addition, it is good to think about the future, since fish require more space to reproduce. There are territorial species that claim a part of the space if they are in the reproductive stage, which could cause stress if the tank does not meet their requirements, and could even increase the level of violence if you have several fish of different species.

4. Relationships between species

You must not forget that the aquatic world has hierarchies and predominant animals, so not all species have the appropriate characteristics to live in the same fish tank. For example, the cichlid family of fish are usually very territorial, so they will eat small fish, they may even eat small objects if they fit in their mouth.

Likewise, fish with a peaceful nature will be constantly stressed by the presence of other fish with an aggressive behavior, so you should know the character of each one if you plan to form a complex aquarium.

5. Food

Feeding is completely up to you, as most fish do not have the ability to stop when they are full, but will continue to eat if you keep feeding them. Consequently, your body will eventually fail from excess food.

How to know if the fish are stressed? 

When a fish feels anxious or overwhelmed, it can develop physical changes and behavior quite different from usual, for which it would be necessary for us as owners to be very attentive. The most noticeable changes may be in the color of their scales, breathing, swimming speed and it is possible that their appetite is also altered.


Your fish usually breathes calmly and deeply. However, being nervous could go to the surface and breathe continuously, the more you do it, the worse you feel, so that in a prolonged time you could die. This is mainly because your pet is having serious problems getting oxygen. Likewise, stress is also capable of producing redness in their gills and that they open more than normal, which is a consequence of agitated breathing.

Respiratory problems prevent the proper arrival of oxygen to the brain and lead to a slow recovery, in the case of noticing the situation in time to save the fish.

swimming speed

The fish are slow and peaceful swimmers, so seeing them swimming hyperactively or with sudden movements is a clear sign of anxiety. In the same way, there is the case of fish that are capable of greatly reducing their movements, to the point that they seem stopped in the water, which could be caused by a high level of tension in their environment.

scale color

If your fish is under constant stress and you haven’t noticed it yet, then it will start to change its scales, where it could lose its color intensity and shine. In some cases, the scales even fall off, normally this is due to the low tolerance it has for some property of the water.


If the fish is stressed, this could cause it to stop eating or eat less enthusiastically, causing illness in a short time. In this sense, it is recommended that you eat at a fixed time and responsibly, by this we mean providing adequate nutrition, depending on the type of fish and its size. It is also advisable to review the characteristics of the water to define the cause of the stress.


Sometimes the fish stays hidden for a long time, so it could take days for you to see them. This may be because there are not enough places to hide and you feel unsafe in the water. Likewise, very intense lighting can also be the reason why the fish decides to hide, so it is advisable not to illuminate the tank excessively.

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