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Stress is the root cause of almost all disease and health problems of aquarium fish. Today we recognize that the health of any living organism is directly related to the level of stress inflicted upon it; for fish this is a major problem because the fish cannot do anything to reduce or eliminate it—they can only fight it or succumb to it. Our fish are confined to the small space of their aquarium, and only the aquarist can control their environment. In a very real sense, we are directly responsible for any and all stress inflicted upon the fish. Later we’ll consider how this occurs, but before that we must understand what stress is and how it harms our fish. Here is how Biology Online defines stress:

The sum of the biological reactions to any adverse stimulus—physical, mental or emotional, internal or external—that tends to disturb the organisms homeostasis; should these compensating reactions be inadequate or inappropriate, they may lead to disorders.

Homeostasis is defined as “the tendency of an organism or a cell to regulate its internal conditions, usually by a system of feedback controls, so as to stabilize health and functioning, regardless of the outside changing conditions.” Physiological homeostasis, or physical equilibrium, is the internal process animals use to maintain their health and life: “the complex chain of internal chemical reactions that keep the pH of its blood steady, its tissues fed, and the immune system functioning” (Muha, 2006).

Four important body functions of homeostasis are closely associated with processes in the gills: gas exchange, hydromineral (osmoregulation) control, acid-base balance [pH] and nitrogenous waste excretion [ammonia]. These processes are possible because of the close proximity of the blood flowing through the gills to the surrounding water, as well as the differences in the chemical composition of these two fluids (Bartelme, 2004). Each species of fish has evolved within a specific environment—and by “environment” in this context we mean everything associated with the water in which the fish lives—and the physiological homeostasis only functions well within that environment. This greater dependence upon their surrounding environment is why fish are more susceptible to stress than many other animals (Wedemeyer, 1996).

How Stress Affects Fish

Stress is caused by placing a fish in a situation which is beyond its normal level of tolerance (Francis-Floyd, 1990). Stress makes it more difficult for the fish to regulate the normal day-to-day physiological functions—the homeostasis—that are essential to its life. Dr. Cliff Swanson, associate professor at North Carolina’s College of Veterinary Medicine, says that stress creates “a fundamental physiological shift in fish, from energy storage to energy usage—the fight or flight response” (Muha, 2005). The survival of any organism depends upon its ability to keep its internal chemical balance from fluctuating too much. When critical energy is being used to fight stress, it is diverted away from other functions. The fish must then work much harder just to “keep going.” Laura Muha (Muha, 2006) likens this to driving a car up a steep hill: it takes more gas (energy) and effort to maintain the same speed as on level ground (level being the norm for the fish).

The effects of stress on fish are very complicated physiologically, and are often subtle. There may or may not be external signs discernible to us—it can continue for weeks and even months, sometimes up to the point when the fish just suddenly dies. The reasons for this are involved.

Adrenaline released during the stress response increases blood flow to the gills to provide for the increased oxygen demands of stress. The release of adrenaline into the blood stream elevates the heart rate, blood flow and blood pressure. This increases the volume of blood in vessels contained within the gills, increasing the surface area of the gills to help the fish absorb more oxygen from the water. The elevated blood flow allows increased oxygen uptake for respiration but also increases the permeability of the gills to water and ions. This is what is known as the osmorespiratory compromise (Folmar & Dickhoff, 1980; Mazeaud et al., 1977). In freshwater fish, this increases water influx and ion losses. This is more critical in small fish than larger due to the gill surface to body mass ratio (Bartelme, 2004).

Short-term stress will cause an increase in heart rate, blood pressure, and respiration as described in the preceding paragraph. The fish can only maintain these altered states for a short and finite period of time before they will either adapt or (more often) the stress will become chronic. During this initial stage the fish may look and act relatively normal, but it is depleting energy reserves because of the extra physiological requirements placed upon it. At the chronic stage the hormone cortisol is released, which is responsible for many of the negative health effects associated with stress.

One of the most characteristic aspects of stress in fish is osmoregulatory disturbance, which is related to the effects of both catecholamine and cortisol hormones. The extent of the disturbance following stress depends upon the ionic and osmotic gradients (difference) between the internal fluids of the fish and its surrounding environment (water)—something we will explore in more detail later. If the stress is persistent and of sufficient intensity, changes in the cellular structure of the gills may occur under the influence of cortisol. In this situation, increased death and turnover rates of branchial epithelial cells leads to accelerated aging of the gills. These degenerating and newly-formed gill cells do not function normally, which further limits the fish's ability to maintain water and ion homeostasis under stressful conditions. Thus, acute stress limits the fish's capacity to osmoregulate, and prolonged periods of extreme stress may result in osmotic shock and death (Bartelme, 2004).

Chronic stress impacts negatively on fish growth, digestion, and reproduction. It is the main cause of deterioration in the slime coat. It significantly lowers the ability of the immune system to respond effectively and fully. And in all cases—stress reduces the fish’s lifespan.

The slime coat or mucus layer is a physical barrier that inhibits entry of disease organisms from the environment into the fish. But it is also a chemical barrier because it contains enzymes (lysozymes) and antibodies (immunoglobulin) which can kill invading organisms. Mucus also lubricates the fish which aids movement through the water, and it is also important for osmoregulation (Francis-Floyd, 1990).

This deteriorated slime coat, along with a lowered immune response, is what allows parasites, bacteria, pathogens, protozoans and fungi to infect the fish and cause disease. More water enters the cells, further impacting osmoregulation and increasing work for the kidneys. The effects of stress on the immune function can linger for some time after other physiological changes have returned to pre-stress levels (Maule et al., 1989).

There are really only two requirements for a disease to occur in a fish: first, the pathogen must be present; and second, the fish must be under sufficient stress to weaken the immune system to enable that pathogen to actually cause an infection. The very few pathogens that can cause an infection without any stress in the fish are quite rare. Clearly, the best disease prevention is to eliminate stress as much as possible.

[continued in next post]

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The Causes of Stress

As was mentioned at the start of this article, stress for fish is a major problem, because the fish cannot do anything to reduce or eliminate it—they can only fight it. Only we can control their environment, so we are directly responsible for any and all stress inflicted upon the fish in our aquaria. This will become very obvious as we consider the many origins of stress—and these are everything outside the required environmental or behavioral needs of the particular fish species that includes:
1) Inadequate tank size;
2) Overstocking of tank;
3) Harassment and aggression from other fish;
4) Improper nutrition;
5) Low oxygen levels;
6) Disturbance of the tank (noise, ambient room factors, tank interior disturbance);
7) Netting and transporting fish; and
8) Environmental factors that include
i. elevated ammonia, nitrite and nitrate;
ii. water parameters that are outside the fish’s natural habitat—GH, pH, temperature, salinity—or a sudden and/or significant fluctuation in any one of these;
iii. lack of hiding places, dependent upon species’ requirements;
iv. overhead tank lighting if bright;
v. water current—too much or too little, depending upon the fish species; and
vi. lack of enough fish to provide schooling according to the species.
Some of these are so self-evident we needn’t discuss them; but the environmental factors in particular are frequently misunderstood and sometimes ignored to the detriment of the fish.

Each species of fish has evolved over thousands of years to live in a particular environment. The water chemistry along with the environmental factors of the habitat are crucial not only to the life of the fish but to the state of its health during that life. As we learned above, the fish’s homeostasis only functions well within the species’ natural environment. A normal lifespan is virtually impossible if the fish’s environmental needs are not met to some extent. For instance, if one intends to house tropical forest fish, “the chemical and physical properties of aquatic environments associated with rainforests must be duplicated, or at least approximated, in order to keep these fishes in the best of health” (Weitzman et al., 1996).

Ammonia, nitrite and nitrate are the lethal forms of nitrogen that are or may be present in all aquaria. Most aquarists understand that elevated levels of all three can be critical, but the fact is that any level above zero for all of these does affect the fish to varying degrees. Ammonia and nitrite cause irreversible harm to the fish, from which they never recover. Nitrate much above the level in the fish’s natural habitat—which in most cases is so low it cannot be measured—does cause stress. While elevated nitrate is not lethal in itself like ammonia and nitrite, it can contribute to a fish’s ill health and premature death by adding to the stress from other factors. A balanced aquarium (fish stock to water volume) along with live plants and regular (weekly at minimum) partial water changes (30-50% depending upon circumstances) should keep ammonia and nitrite at zero and nitrate as close to zero as possible.

Water parameters play a vital role in preventing stress. We know that many common species seem to manage in an array of differing water parameters—but surviving is very different from living a normal and healthy lifespan. The physiology of each fish species is designed by nature to operate within a specific range of water parameters, meaning general hardness [total dissolved solids], pH and temperature. The physiological equilibrium mentioned previously only operates properly within specific ranges (Muha, 2006). When the parameters extend beyond these ranges, the fish must expend considerably more energy just to “keep going” and this results in chronic stress that further wears down the fish.

Water is continually entering the fish’s cells via osmosis, and to prevent those cells from exploding, the fish controls the water flow through osmoregulation, which is a complex series of chemical processes. The pH of the tank water affects the pH of the fish’s blood, so another complex series of chemical reactions must occur to control the pH of the blood. The pH also affects the fish’s ability to transport oxygen through the blood to its tissues, and changes to the blood pH affect the ability of the fish’s hemoglobin to hold oxygen (Muha, 2005).

The total dissolved solids are another major factor. This not only includes common general hardness (calcium and magnesium mineral salts that make up the GH) but the dissolved solids that enter via fish foods, water conditioners, pH adjusters, medications, additives like salt, and others. This is one reason why a regular significant water change—which is the only way to remove these TDS—is so important for fish health.

A change in temperature beyond the preferred range for the species alters the rate of all the afore-mentioned chemical reactions within the fish’s body, plus the rate they metabolize food; temperature also impacts the level of oxygen in the water which in turn affects the amount of oxygen that can be transported by the hemoglobin in the blood.

The physical environment is also extremely important to fish health. Some species need multiple hiding places; some cannot tolerate bright overhead light, and/or a light coloured substrate; and some fish wear themselves out battling continual water currents that are greater than what they are designed for, while others may need these. Shoaling or schooling fish need a sufficiently-sized group; in lower numbers they are under constant stress due to insecurity or lack of interaction that may have social/hierarchical relevance, or both. Aggressive fish in the same tank will keep other fish constantly under fear of attack, even if no actual physical confrontation occurs. As all this illustrates, there are indeed many aspects to consider for true compatibility and truly healthy fish in a community aquarium.

From all of this we can gather one absolute: preventing stress is probably the most important aspect of responsible fish keeping.


Bartelme, Terry D. (2004), “Short Take: Stress In Fish, Part II: Why You Should Care About Stress In Fish,” Advanced Aquarist online:

Drs. Foster & Smith LiveAquaria, Stress and Fish Health:

Evans, Mark E. (2004), “The Ins & Outs of Osmosis,” Tropical Fish Hobbyist, February 2004.

Francis-Floyd, Ruth (1990), “Stress—Its Role in Fish Disease,” CIR919 (December 1990), Fisheries and Aquatic Sciences Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.

Maule, A.G., R.A. Tripp, S.L. Kaattari, & C.B. Shreck (1989), "Stress Alters Immune Function and Disease Resistance in Chinook Salmon (Oncorrhynchus tshawytscha)," Journal of Endocrinology, 120, 135-142, 1989.

Muha, Laura (2005), “Stress” in “The Skeptical Fishkeeper” column, Tropical Fish Hobbyist, December 2005.

Muha, Laura (2006), “Fish Growth vs. Tank Size” in “The Skeptical Fishkeeper” column, Tropical Fish Hobbyist, December 2006.

Narten, Thomas, “Fish Stress and Healthy Fishkeeping,” The Aquaria FAQ at

Wedemeyer, G.A. (1996), "Transportation and Handling," in Principles of Salmonid Culture, W. Pennell and B.A. Barton (eds.), pp. 727-758. Cited in Bartelme (2004).

Weitzman, Stanley H., Lisa Palmer, Naercio A. Menezes & John R. Burns (1996), “Maintaining Tropical and Subtropical Forest-Adapted Fishes,” Tropical Fish Hobbyist, June 1996.

Byron Hosking
April 15, 2012
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