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Are fish stressed by cats?

Are fish stressed by cats?

Most fish can tolerate environmental conditions that differ somewhat from the natural conditions in which they evolved. This does not mean, however, that they will be as healthy or live their full normal life span. For example, keeping a fish in water that is cooler (or warmer) than its preferred condition forces its body organs to work harder to keep it alive. That is, such conditions place the fish under increased stress.

Increased stress reduces a fish’s ability to ward off diseases and heal itself (e.g., if its fins get nicked, or parasites get introduced into the tank with newly purchased fish). In addition, stress reduces a fish’s ability to breed successfully and shortens its natural life span. A small amount of stress by itself is not usually fatal, but as stress levels increase, a fish’s ability to cope with it decreases. Thus, one of the most important goals of a fishkeeper is to remove sources of stress wherever possible.

It should be noted that eliminating stress does not guarantee that your tank will be healthy. But it significantly increases the odds. Many netters boast regularly about how they’ve kept fish (apparently) «healthy & happy» for long periods of time under (apparently) highly stressful conditions. Such aquarists are sitting on a time-bomb; the not uncommon followup story will refer to one fish getting sick, then another, with an end result of multiple fish deaths. Reducing stress simply increases the likelihood that a tank will stay healthy (much the same way as eating right, exercising, getting the proper amount sleep is generally associated with a long healthy life for humans).

What are the common factors that lead to stress in aquariums?

In the following, we list some of the more common stress-inducing conditions. In all cases, the level of stress induced by a specific factor is highly species-dependent. Thus, an aquarist is advised to be aware of the type of stress that will be present in their tanks and select fish known to tolerate such conditions well. For example, if your water is hard and alkaline, you’re best off selecting fish that thrive under such conditions.

Nitrogen compounds (ammonia, nitrite and nitrate) have varying degrees of toxicity and are stressful at all levels. Ammonia is toxic in low concentrations. In ANY concentrations, ammonia severely stresses fish. Consequently, a healthy aquarium must have an adequate biological filter that quickly converts ammonia to nitrite (and nitrate). Although significantly less toxic than ammonia or nitrite, nitrate also stresses fish. Thus, a means of removing excess nitrate (e.g., through regular water changes) helps keep an aquarium healthy.

The water temperature of your tank should match the needs of its inhabitants. Keeping water temperature too cold or too warm for a particular species will stress those fish. For example, goldfish prefer cooler temperatures (less than 70F) than most tropical fish (goldfish survive winters in ponds where temperatures approach freezing), guaranteeing that a tank containing both goldfish and tropicals will either be too cold or too warm for some of the inhabitants.

Some fish prefer soft water, others prefer hard water. Keeping a soft-water preferring fish in harder water (and vice versa) is stressful.

Some fish prefer acidic water, some prefer alkaline water, others prefer water with a neutral pH. (Some fish don’t care too much.)

Some fish live in brackish water conditions; they will do better in water with a small amount of added salt. Other species are extremely intolerant of salt. Add salt only if all of a tank’s inhabitants can tolerate salinity. Mollies, for example are known to like salt, whereas many species of catfish tolerate no salt at all. In general, fish lacking scales (or having small scales) don’t tolerate salt well.

The amount of physical space required for a particular fish depends on its species. Some fish do just fine in a 10g tank, others need 100g or more. Keeping a fish in a tank that is too small for it increases the level of stress (on everyone), frequently leading to increased aggression among tank inhabitants. Note also that the amount of space required may change should fish pair off to breed. Breeding cichlids, for example, claim a portion of the tank for themselves, chasing away any fish that encroach on its territory. Thus, the onset of breeding behaviors frequently increases stress levels.

Not all species of fish mix well with others. As an obvious example, most cichlids will eat smaller tank inhabitants (e.g., anything they can fit in their mouths). Even if too big to be eaten, however, peaceful fish will be stressed if kept with aggressive fish that chase them around all day. Moreover, many fish communicate through behavior and body language (e.g., cichlids frequently establish a «pecking order» in which one fish is king). Fish of one type of species may not recognize the signals given off by others, guaranteeing continual strife.

Some fish school in nature, spending their entire lives in large groups (rather than individually); they never feel comfortable or «safe» when kept by themselves. Cory cats for example, do better in a tank with 6 or more other Corys than they do by themselves. While it may be tempting to buy six different kinds of fish, this may not be ideal for the fish themselves. The opposite can also be true. Some fish are more aggressive towards members of their own species (e.g., mating behaviors), whereas they may not feel threatened by other species and pretty much ignore them.

Fish need oxygen, and some fish are more tolerant of low-oxygen water than others. Water with insufficient oxygen stresses fish. Note that as the water temperature goes up, the amount of dissolved oxygen in water decreases.

Poor nutrition also causes stress. A healthy diet is a varied diet, and one should avoid using old foods in which vitamins and other nutrients have broken down. «Old food» includes food that has been stored in hot places, been exposed to air (not sealed), etc.

The «cure» of adding medicines to tanks is often worse than the original disease. Medications that kill bacteria, parasites, etc. are usually not too discerning: they may also kill your nitrifying bacteria (now you REALLY have a major problem) or be toxic to the fish themselves. For example, some species of fish do not tolerate certain types of medicines at all. Adding such medications may weaken healthy fish to the point that they become susceptible to the original disease.

Adding untreated water to your tank may introduce chlorine or chloramine, both of which are toxic to fish. Be sure to treat all water prior to adding it to your tank.

Sudden change in water conditions can be stressful. Within limits, most fish can adjust to sub-optimal water conditions (e.g., wrong temperature, wrong pH). However, fish have difficulty adjusting to a SUDDEN change in water chemistry. Thus suddenly raising (or lowering) the temperature, changing the pH, changing the water hardness, etc. stresses a fish. It is more important to keep the water chemistry stable over the long haul than keeping water conditions exactly optimal.

In summary, many factors lead to fish stress. Minimizing and eliminating sources of stress increases the chances of keeping tank inhabitants healthy. The exact amount of a stress individual fish can take depends greatly on what species it is, its age and size, etc. A stressed fish is a weakened fish. Although it may appear healthy to the casual observer, it will be more susceptible to disease, injury, etc. In contrast, healthy (unstressed) fish will be able to ward off disease and infection on their own. Thus, the appearance of disease in a tank is frequently brought on by «poor water conditions» that leave fish with weakened immune systems.

How can I tell if my fished are stressed?

In short, stressed fish don’t «act normal», with «normal» defined according to the species of fish. Once you’ve had fish for a few weeks, you’ll see that each species behaves in its own characteristic way (that’s why fish are fun to have!). Some fish tend to always stay near the top of the water, others near the bottom. Some fish swim continuously, others stay in one place. Deviation from that norm usually indicates stress.

  • Fish stays near the surface gasping for breath, indicating that it has trouble getting enough oxygen (the concentration of dissolved oxygen is highest near the water’s surface). Possible causes include low oxygen concentration due to poor water circulation, toxins that have damaged its gills, high ammonia or nitrite levels, etc.
  • Fish won’t eat, or doesn’t eat as aggressively as in past.
  • Fish stays hidden continuously and won’t come out where it can be seen. Possible causes: aggressive fish, insufficient cover (e.g., plants, wood, etc.) to make fish feel «safe» while swimming about.
  • Fish has nicked fins, open wounds that don’t seem to heal. Possible cause: fish is target of aggression. Normally, minor nicks and cuts heal quickly. If they don’t, stress levels may be suppressing the fish’s immune system.
  • Fish has disease (parasites, fungus, etc.) Of course, the disease itself is a major problem. But in most cases, a healthy fish’s immune system keeps it from getting sick in the first place. Thus, getting sick is a sign that the fish is in a stressed state (or had been until recently).

The Effects of Stress in Fish

The Effects of Stress in Fish

Stress is a critical factor in fish health. It is so important, in fact, that scientists have studied it in detail, both in the wild and in captive fish. Stress is a very complicated subject that permeates every aspect of fish-keeping.


The primary rule to remember with stress is that, as the saying goes, prevention is better than cure. The things that cause stress in a fish’s life (“stressors”) include overcrowding, handling, a poor or unfavorable environment, inappropriate or aggressive fish sharing the same tank and, in the wild, predators. All of these (and others) cause fish to react in different ways depending on the type and amount of stress.

Fish have evolved and live in a relatively stable environment. Their stress responses are consequently better at handling short-term trouble and are not as well-suited to long-term environmental stressors. Unfortunately, both types of stress can cause problems.

Short-term Stress

For short-term stress, the most common reaction is one everybody recognizes — to flee from danger. In the wild, the cause is frequently a predator. In captivity, it can be a net that causes the reaction, as the keeper tries to capture a fish for closer examination or transfer to another tank.

When a fish senses this kind of danger, it triggers a short-term alarm reaction by releasing hormones, including adrenalin for its locomotory muscles. This will give it a shot of energy to escape quickly. The fish also releases cortisol. Problems occur because the fish’s body exchanges long-term health for a short-term boost to relieve the cause of stress — the adrenalin disturbs the fish’s natural osmoregulation (the balance of salt and water in its body) and the cortisol affects white blood cells and reduces the effectiveness of the immune system.

Once the panic has passed, the fish must also regain its natural balance. This can take hours or days, even after only a short period of stress.

Long-term Stress

Long-term changes, such as a poor or unsuitable environment, are handled with the same initial response – an alarm message to escape. However, if escape is impossible, the fish does not stop being stressed: it begins to adapt to the new environment as best it can.

At first, the fish’s body tends to overreact but, with time, it will adapt to reach the best possible balance – physiologically and behaviorally. Throughout the period of adaptation, the fish still prioritizes reacting to the new environment and remains stressed, so its immune system suffers and it is prone to disease. Adaptation normally lasts from four to six weeks.

However, if the fish continues to be in stressful conditions, such as a constantly deteriorating environment or endless bullying from aggressive tankmates, it continues to try to adapt and extends all the bodily responses as long as necessary. This reduces its chance of survival. In the worst possible situation, where adaptation to the new environment is impossible (such as putting marine fish in fresh water), the fish will exhaust itself fatally.

As a fishkeeper, it is extremely important to consider the effects of stress. Planning ahead, careful control of the environment and management of the fish population are fundamental basics in fishkeeping. Less stress means less disease.

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