Causes and Signs of Stress in Fish

The constant advance of ichthyological studies prove that fishes 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 dogs and cats would do. Therefore, in this article we show you some of the signs that can 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 may have serious and harmful consequences on their health, however, the causes may be simpler than in complex organisms, as they are usually related to the environmental conditions that exist in the fish’s environment.

In this sense, many people do not know that the 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 the fish, among many other disadvantages that we can find. For these reasons, the following 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 may be toxic to a greater or lesser extent to fish. Such is the case with ammonia, which is naturally formed by fish waste and food waste, but which in all circumstances is contaminating to the fish and could cause high levels of stress. For this reason, an optimal fish tank must mainly have a filtering system, to dilute and quickly transform the different contaminating substances in the water.

2. Water conditions

The temperature should be according to the type of fish, since maintaining a very low or high temperature is another likely cause of stress. A common example is that of goldfish, which can live peacefully in temperatures up to 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 into 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 who are salt intolerant usually have no scales or very small ones.

3. Size of the fish tank

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

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

4. Relations between species

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

Also, fish of a peaceful nature will be constantly stressed by the presence of other fish of aggressive behavior, so you must 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 because of excess food.

How do you know if the fish are stressed?

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


Usually your fish has a calm, deep breath. However, when you are nervous, you may go to the surface and block continuously, the more you do so, the worse you feel, so you may die over time. This is mainly because your pet is having serious problems getting oxygen. Stress can also cause your gills to become red and open wider than normal, which is the result of heavy breathing.

Breathing problems prevent oxygen from reaching the brain properly and lead to a slow recovery, if the situation is noticed in time to save the fish.

Swimming speed

Fish are slow and peaceful swimmers, so seeing them swimming in a hyperactive manner or with sudden movements is a clear sign of anxiety. Similarly, there is the case of fish that are capable of greatly slowing down their movements, to such an extent that they seem to be stopped in the water, which could be caused by a high level of stress in their environment.

Colour of the scales

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 the intensity of the color and its brightness. In some cases, even the scales fall, usually due to the low tolerance to some property of water.


If the fish is stressed, this could cause it to stop eating or to eat less enthusiastically, causing illness soon after. In this sense, it is recommended that you eat at a fixed time and in a responsible manner, by this we mean to provide adequate nutrition, according to the type of fish and its size. It is advisable to check the characteristics of the water to define the cause of the stress.


Sometimes, the fish stay hidden for a long time, so it could take days for you to see it. This may be because there are not enough places to hide and you feel unsafe in the water. Very intense lighting can also be the reason why the fish decides to hide, so it is advisable not to over-light the fish tank.




Mehul D

I’m Dedhia. I’m a computer science engineer from India who lives in Boston, MA. I love dogs and cats, traveling, watching movies and listening to music. I believe it is important to maintain a healthy lifestyle by eating well and exercising regularly. I founded the website which provides pet owners with information about buying the best products for dogs and cats.