Disease Prevention & Control
John Gratzek presented an overview of the diseases of ornamental fishes in 1980. He reported that most (60%) aquarists who discontinue keeping fishes do so because the fishes die. He identified common problems with keeping ornamental fishes and stated that the treatment of diseased fishes is the most common problem. For new hobbyists this can be a major problem compounding the fact that they have often not researched the necessary information to be successful in their new hobby. Initial purchases of fish are often done for aesthetic rather than practical reasons and regularly, newly bought fish will often die resulting in the hobbyist losing interest in ornamental fish altogether. Those that get past this initial stage often find an appetite for becoming better informed and often acquire greater knowledge than some “experts”.
The increasing popularity of rainbowfishes has resulted in a significant increase in the number of commercial operators breeding, rearing and distributing rainbowfishes. This increases the potential for dissemination and exacerbation of infectious diseases. 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 aquarium environment.
A wide range of pathogens (bacteria, parasites, etc), environmental factors (water quality, etc), and even aquarium keeping methods, can cause disease problems in aquaria. Often these factors are linked in disease outbreaks. Mismanaged aquariums allow opportunistic organisms to cause disease. For example, a decline in water quality associated with poor aquarium keeping practices may lead to an increase in the incidence of bacterial infections. Another important source of pathogens is new fish obtained from other hobbyists and suppliers, both these sources need to be appropriately screened to ensure that introduction of pathogens to your aquarium is minimised.
Despite what you may read elsewhere, reproducing the intricacies of the natural aquatic ecosystem of rainbowfishes in an aquarium is not only a very difficult task, but almost impossible to accomplish with overall success. It is not as simple as having the water at a particular temperature or pH. Water chemistry (hardness, nitrates, alkalinity), suspended particles, levels of oxygen/CO2, parasites, prey and predators, symbionts, wet and dry seasons, light (intensity, cycle, spectrum), droughts, floods, water flow, substrate, vegetation, woody debris, sound, space, shelter, etc., etc., are all variables that are either difficult to replicate or are difficult to control in a captive environment. In consequence, something is likely to go wrong—at the very least occasionally— making the captive aquatic animals become chronically or acutely ill. Opportunistic bacteria and parasites can cause dermal and systemic infections. Intestinal nematodes can cause chronic wasting (anorexia) and considerably damage to the intestinal lining. Additionally, water moulds and fungi may also present disease problems in poorly managed aquariums.
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 skin has been damaged. Most fish pathogens fit this description and are very opportunistic rarely causing disease in healthy wellmanaged aquariums.
Fish disease usually occurs as a result of adverse interactions between the fish and their environment. The natural environment of rainbowfishes is vastly different from that of an aquarium. Most aquarium environments are completely devoid of structure. They are generally featureless, monotonous enclosures with little opportunity for the inhabitants to display any natural behaviour. They bear no resemblance whatsoever to the fish’s natural environment and densities can be up to 100 times greater than those in nature. Reductions in density alone will produce healthier fish. When fish that are already over-crowded in the aquarium are further stressed; for example, by inappropriate aquarium conditions, nutritionally inadequate feeds, or the nature of captivity itself, their natural immune system may be weakened and the ability of the fish to protect itself against infectious diseases is reduced.
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. There are publications with in-depth chapters on disorders associated with general ‘stress factors’ in fish. 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.
The concept of biological stress was first introduced by Hans Selye who described it as the non-specific responses of the body to any demand. The idea for stress arose from the concept of homeostasis: a relatively constant steady state maintained within certain tolerable limits and the concept of a milieu intérieur or internal environment of the body in which cells are nourished and maintained in a state of equilibrium. Since its original introduction, the concept of stress has evolved and our ideas about stress continue to change. More recently stress has been described as the physiological cascade of events that occurs when an organism is attempting to resist death or reestablish homeostatic normality in the face of a perceived threat.
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 prepathological state that makes it vulnerable to disease, abnormal behaviour, reduced growth, reproductive failure, immunosuppression or some other type of undesirable shift in biological function. Although a stress response is necessary to enable an organism to adapt and overcome the threat, under conditions of chronic stress, the adaptive value of the stress response is typically lost, as indicated by reduced health and fitness.
Identified stressors in fish are numerous and include sudden and extreme changes in the physical environment (e.g., temperature and water chemistry), animal interactions (e.g., predator avoidance, being kept with the wrong species or individuals, aggressive interaction, parasitism, competition for space, and spawning), aquarium practices (e.g., netting, handling and transport) and water quality (e.g., high ammonia, organic pollutants, etc.). Light (excessive or rapid changes in intensity), noise and other disturbances can also stress fish and should be minimised. Rainbowfishes will often dash frantically about the aquarium injuring themselves, in response to normally harmless stimuli such as turning on the fishtank light in a dark room.
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 window. 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.