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The information provided herein is intended as a guide to the most common disease problems of rainbowfishes maintained in aquaria or ponds. However, this information should not be substituted for consultation with an experienced fish health professional.
The increased interest in keeping rainbowfishes has also increased awareness of and experience with pathogens that affect their health, growth, and survival. Rainbowfishes maintained in aquariums and ponds are entirely dependent on our management of their environment to maintain their health. Health or disease is a complex balance between pathogens, the fish or other organisms and the environment. Mismanaged environments stress fish and allow opportunistic organisms to cause disease.
Many diseases can be eradicated or kept to minimal levels by careful management and a good knowledge of the basic biology and natural history of the species (both fish and pathogen) concerned. Therefore, disease prevention and control are a significant part of the rainbowfish hobby. For this reason, it is extremely important that you read and understand a practical book on this subject. Several excellent choices are available and they represent one of the best investments in time and money for the serious rainbowfish keeper.
Diseases causing organisms fall into two categories:
Obligate Pathogens: always cause disease when able to invade the host's bodies. They may however, only cause disease in certain circumstances.
Facultative Pathogens: don't usually cause disease but can when their population explodes due to environmental conditions or for instance because a fish's immune system is depressed, the water quality is diminished or the fish is wounded. Most fish pathogens fit this description and are very opportunistic relatively rarely causing disease in healthy well-managed aquariums.
A variety of parasites and pathogens can and do infect rainbowfishes. Most are naturally present in low numbers and normally do not cause problems. However, disease is rarely a simple association between a pathogen and a host fish. Usually other circumstances must be present for active disease to develop in a population. These circumstances are generally grouped under the umbrella term "stress". There is a wealth of scientific research suggesting that captive fish populations suffer from stress. Stress is the response of the fish to a 'stressor' (external or internal) and is a normal feature of life, serving important adaptive functions. The stress response consists of a combination of four general biological responses: behavioural, autonomic, neuroendocrine and immunological. The nature of this biological response varies between individuals and is influenced by factors such as previous exposure, genetics, age and physiological state.
Regardless of the combination of biological responses, the result is an alteration in the fish's normal biological function as it attempts to adapt to or cope with the stressor, behaviourally and/or physiologically. In most cases, this altered biological function has a minimal effect on the fish's wellbeing; the stressor is either brief or it is eliminated, so biological function soon returns to normal. However, if the stress is not alleviated or if the stressor is large enough, the fish is forced into a pre-pathological state that makes it vulnerable to disease, abnormal behaviour, reduced growth, reproductive failure, immunosuppression or some other type of undesirable shift in biological function. During this time, the fish experiences distress, and its wellbeing is threatened.
Identified stress factors that affect individual fish and fish populations are numerous. They include sudden changes in the physical environment, competition for space (crowding), being kept with the wrong species or individuals, aggressive interaction, unsatisfying food, spawning, handling and water quality (e.g., low pH, high ammonia or organic pollutants). Light (excessive or rapid changes in intensity), noise and other disturbances can also stress fish and should be minimised.
Although temperature can be considered a stressor, it is considered separately as the effects observed differ from other stressors. Temperature regulates their metabolism and is a major environmental factor in determining growth rate, metabolism, and nutritional efficiency. In fact, temperature will influence all biological and chemical processes in an aquarium. The temperature tolerance of fish can be further defined as stenothermic or eurythermic. Stenothermic species are those that thrive in a very narrow temperature range. Eurythermic fish are those that can tolerate a fairly broad temperature range. In addition to its direct effect on animal health and immunocompetence, many infectious diseases occur in very specific temperature ranges. The growth of parasite populations is also at least in part attributable to how quickly they can complete their lifecycle, which is closely linked to water temperature.
Fishkeeping practises directed at limiting stress are likely to be the most effective in preventing disease outbreaks. Simple measures like increasing filter performance, providing dark backgrounds, natural substrate material, aquatic plants, submerged items such as rocks or driftwood, and overhead cover (floating plants) will improve the aquarium environment. If you want your rainbowfishes to survive and grow, then it is imperative that their aquarium is maintained under conditions conducive to good health. Well-nourished rainbowfishes maintained in highly favourable environmental conditions will be resistant to most pathogens.
Far more rainbowfishes die from inappropriate aquarium conditions rather than disease. Whether by high-tech filtration systems or simply regular waterchanges, good water quality should be maintained. This will promote the immune system function and general health of the fish being kept.
Any time you observe a problem with your fish; always check the pH, ammonia and nitrite levels, and other water parameters. Quite often, a simple water change will result in the recovery of the fish. If treatment is indicated, it will be most successful if it is implemented early in the course of the disease while the fish are still in good condition. It has been estimated that over 80% of diseases reported by hobbyists are caused by poor environment, particularly poor water quality.
Water conditions can kill fish directly, for example if ammonia is too high, oxygen too low, or if city water is not properly dechlorinated. Sublethal water quality problems can contribute to disease indirectly through immunosuppression associated with chronic stress, or more directly by favouring the growth of opportunistic organisms. Conditions that are consistent with poor water quality, such as overcrowding, overfeeding, or inadequate filtration, may favour "blooms" of opportunistic bacteria, protozoa, or fungi. An excellent example is Trichodina, a single-celled protozoan that is notorious for causing problems when fish are overcrowded and water quality is suboptimal.
To begin to understand why disease outbreaks occur, you must first learn about the fish's environment, the stresses placed on the fish by that environment, and the cause, effect, and importance of disease. Since the fishes cannot simply tell us that they are feeling sick or uncomfortable we must educate ourselves as to their normal and abnormal behaviours so that we can have a functioning 'early warning system'. Daily observation of fish behaviour and feeding activity allows early detection of problems when they occur so that a diagnosis can be made before the majority of the population becomes affected. If one is certain that a problem exists the next step is to determine if the problem is related to environmental or biological factors.
© Copyright Adrian R. Tappin
Created July, 2005
Updated December, 2008
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