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Stress In Fish



Something often overlooked and definitely under researched is the effect of stress on goldfish. We often hear that a disease may have been triggered by stress but how is that and why is that? And how can we create a stress free environment for our fish pets? I have tried to find as many answers and gather as much information as possible about stress in goldfish and will set it out here as simply as I can in 3 question and answer parts.

1.What is stress and why does it affect the fish negatively?

Stress is the reaction of a part of the brain called the hypothalamus to a stress-or. The hypothalamus re-acts to the stress-or by beginning a chain of chemical activity that results in the production of hormones from the adrenal glands. In people these are located around the kidneys and in goldfish due to their small compact bodies the glands are merely adrenal tissue inside the kidneys.

When the fish encounters a stressor (more on stressors below), the adrenal tissue produces 2 hormones. The first is epinephrine. This is the hormone that prepares the fish for adversity. It sends blood flowing faster to the brain allowing for the uptake of more oxygen, it speeds up the heart, it utilizes nutrition to provide extra energy to combat immediate threat.

This is all good. But only short term. If the hormone continues to circulate (in conditions of enduring stress) it weakens and exhausts the fish dramatically.

The second hormone that is produced is cortisol. This is a powerful hormone that increases metabolism. It is also dangerous to the immune system of a fish when prolonged. It disturbs and damages the protective immunity process called phagocytosis.

2.What is phagocytosis?

This is the name given to the complex process of cell invasion and neutralization. When a bad bacteria settles on the fish, the good phagocytic cells surround the bad bacteria and begin to ingest and destroy it by releasing sodium hypochlorite which is actually the main ingredient in household bleach. The dead bacteria is then released into the bloodstream and passed out through the kidneys in urine.

Now what happens when there is a prolonged production of the second hormone cortisol is this: The cortisol represses the digestive enzymes of the phagocytic cells and literally destroys their ability to ingest and anihilate the bad bacteria- leaving the fish vulnerable to attack from all kinds of resident bacteria and parasites.

3. What are common stressors in the aquarium?

Here is a list:

Presence of ammonia

Presence of nitrites

Presence of high nitrates

Fluctuating pH

Changing water temperatures


Over crowding of the aquarium space (too many fish)

Bullying or breeding behavior

Lack of correct or adequate nutrition

Overfeeding (food left uneaten)

Too strong light/lights on at night

Too little light

Lack of dissolved oxygen

Ornaments that injure or interfere with swimming space

Too strong a current (some fish, not all!)

Over medication and

Wrongly diagnosed medication.


In conclusion. Our fish are designed to survive. They have strong immunity given the right conditions, especially the correct water environment, space and food. If we aim to avoid all the above stressors we have the chance of being able to raise fish with powerful immunities- their best preventative defense against parasites and bacteria getting a hold in the first place.

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