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Cleanliness is one of the cornerstones of fish health management. Accumulation of organic material, often associated with inappropriate feeding and stocking rates, creates an environment where opportunistic bacteria, fungi, and parasites can flourish. To minimise this, water exchange should be adequate for the stocking densities and feeding rates. Be sure to precondition the water before adding it to your aquarium. Particulate matter (faeces, uneaten food, dead plant material etc.) should be removed on a regular basis. This includes removal of debris by syphoning, manual removal of algae from tank walls, and regular cleaning and removal of particulate matter from the filter. Any sick or dead fish should be removed promptly, as they are an important means of transmitting infectious disease to other fish in the system as well as adding to the organic load if they are left to decay.
Faeces can provide a survival capsule for some fish pathogens. Faeces is made of what remains of a fish's food after digestion, mucus, cells lining the gut that are shed continually, dissolved chemicals such as ammonia and harmless (and even helpful) bacteria naturally found in the gut and pathogens. Lipids (fats) can create a waterproof coating on the outside of the faecal material, thus the contents, including any pathogens present, are at least for a time insulated in a cocoon of material that is conducive to their survival. In recirculation aquarium systems little dilution or dispersion can occur. Thus not only is infection passed to "clean" fish, but the infected fish are subject to re-infection. While a fish may be able to deal with a small dose of a pathogen, if subjected to continual re-infection a disease may be caused. This is especially so as in an untreated recirculation or undergravel system, the amount of the disease causing organism in the water rises and often the water quality falls, both of which make the fish more susceptible to infection and disease.
It is probably wise to regularly empty and disinfect breeding and raising aquariums to try to ensure that pathogens cannot build up in a system. You may choose to do this at the end of a breeding session to ensure any pathogens are not lying in wait to infect new fish placed in the aquarium. When disinfecting a aquarium system, remember to clean all parts including filter and water return lines. It is important that syphon hoses, nets, brushes, and other equipment used to clean tanks should be treated with a sterilising solution when used in different tanks. An easy way to achieve this is to have a plastic container into which equipment can be dipped or placed between uses. Equipment should then be removed, and thoroughly rinsed with fresh water, before being stored dry. Chemicals used should be minimally toxic to fish yet be effective at removal of infectious particles and other organic debris likely to accumulate on equipment.
Solutions of chlorine are effective for disinfecting equipment by submersion at concentrations of 200 mg/L for 30 to 60 minutes. Concentrations of 10 mg/L for 24 hours are effective for disinfections of tanks. However, repeated use and extended exposure of the silicon sealant to high chlorine concentrations will destroy or render the adhesive bond ineffective on glass aquariums with disastrous results. Also, certain materials may deteriorate after repeated exposure to chlorine. Chlorine will dissolve sponge filters and cause mesh nets to rot. Rubber and synthetic or natural fibres may degrade rapidly, but most plastics are unaffected.
If using chlorine for disinfecting equipment or tanks which are not in use, but which are in the vicinity of others housing live fish, the granular form (Calcium hypochlorite) should be used. Granular chlorine does not volatilise as readily as liquid chlorine (Sodium hypochlorite). In a poorly ventilated fishroom, fumes from liquid chlorine can cause fish kills in adjacent tanks. Chlorine fumes can also be harmful to the aquarist. Always wear eye protection and rubber gloves when handling large quantities of chlorine. Chlorine residue can be neutralised by using 7.4 mg/L Sodium thiosulfate for each 1 mg/L chlorine present in solution (7.5 grams of sodium thiosulfate will neutralise the chlorine present in 5 litres of a solution of 200 mg/L).
Commercial sanitisers such as benzalkonium chloride or quaternary ammonium compounds are also suitable and should be used according to label instructions. Sanitisers are compounds effective against all types of infectious agents including bacteria, fungi, viruses and protozoa and differ greatly in their physical, chemical and biocidal properties, mode of action, trade names, composition and availability.
Regular control and monitoring of water quality is imperative and will greatly reduce the likelihood of a disease occurrence. Critical water quality parameters include temperature (particularly sudden and dramatic shifts), dissolved oxygen, pH, alkalinity, hardness, nitrogenous wastes, and toxic substances. Water quality should be monitored frequently and corrective measures initiated if conditions become unfavourable. Aim to keep your ammonia and nitrite at zero levels, and nitrate down to a minimum. Nitrate levels over 20 mg/L (ppm) can cause problems with excessive algae growth and can lead to fish health problems in the longer-term. Optimum water conditions must be maintained at all times. Sub-optimum conditions, while not immediately lethal, may stress the fish, resulting in delayed mortality. Therefore, it is important to become familiar with water testing and have the necessary test kits available.
The use of a good quality food will provide the fish with all the nutrients that they need to remain healthy and to grow. Poor nutritional health can greatly enhance the progression and severity, and reactivation of disease. Rainbowfishes fed a nutritionally complete diet are better able to cope with stress and to resist disease. However, you should note that even good quality food will deteriorate if improperly stored or kept too long. Storage time for most commercial fish foods will vary depending upon environmental conditions; however, as a rule of thumb, 90 days is normally the maximum safe storage time for fish feed. Fish foods should be stored in a cool and dry place (refrigerator), and used within 30 days of opening. Never feed mouldy, discoloured or clumped feed. Moulds on feed may produce aflatoxins, which can kill fish.
Diets consisting of material derived from the wild, such as fish fillets or wild caught foods such as daphnia, mosquito larvae etc., are often perceived as having great nutritional benefits as they are believed to contain many macro nutrients essential to the good health of the animal. However, there is an inherent risk in using "wild diets" as they may introduce pathogens to the aquarium fish. These may take the form of parasites that use an intermediate host to enter their final host, or simply be concentrated by the feeding of the prey item to form an infectious particle which when ingested can establish the infection. Potentially the highest risk here is the use of wild fish in diets which can, under appropriate conditions, be a very high risk factor allowing large amount of pathogen to enter the diet if the source material was infected. Plankton samples collected from the wild can carry diseases or parasites. Artemia cysts can carry bacteria such as Vibrio.
Biofiltration
It is not the aim of this section to explain how filters work to improve water quality. However, it is worth reiterating the point that the stress caused by poor water quality does create the conditions in which pathogens are likely to cause disease. Fish kept in good quality water are better able to resist pathogen invasion because it is more likely that their immune system is in better condition. That said, biofilters can act as a means of reducing pathogen loads but care is required as they can also harbour pathogens.
A well established biofilter can reduce the population of pathogens in an aquarium or pond system. The reasons for this are very complex and poorly understood; however, one of the most important consideration is the interaction between the bacteria and protozoa that colonise a biofilter. The normal flora of a biofilter consists of a well established population of bacteria and a varied population of protozoa either living in, on or in close association with the filter media. These organisms are very well adapted to their environments and:
- Produce enzymes that digest other bacteria and viruses from the water column.
- Feed on bacteria or viruses directly (in the case of protozoa).
- Produce aggressins that mop up micro-nutrients from the surrounding environment, for example chemicals known as siderophores, take up iron directly from the environment. Aggressins are released to starve competing organisms.
- Produce natural antibiotics to prevent the new microbe becoming established.
They also have two other minor roles acting as a:
- Reservoir for bacteriophages (a type of virus) that can kill pathogenic strains of bacteria.
- Mechanical filter trapping bacteria, viruses and parasites either in the media or on the biofilm.
The biofilm is a very hostile environment to new bacteria, each bacterial species that has already colonised the filter is competing for nutrients with each other; less aggressive species starve to death and, in turn their organic components are recycled. In addition to this the protozoa are consuming microbes continuously and again their waste and any dead protozoa will be recycled. All of these defence mechanisms used by the established bacteria can make it a slow process for a new species of microbe to become established in a filter. An obvious example of this is the long period of time it takes for a biological filter to mature from the ineffective filters colonised initially by bacteria such as Pseudomonas spp. to a bacterial flora dominated by Nitromonas, Acetobacter and Nitrosomas species. In an established system a mature biological filter can significantly reduce the level of circulating pathogens.
As always there has to be a word of warning with this approach to reducing pathogen load. The pathogen may become established in the filter media, which ultimately acts as a reservoir continually shedding the pathogen into the environment. This can happen when the load of pathogenic bacteria is so high that it out-competes the established bacteria in the filter. The pathogen will have its own suite of aggressins that it uses to survive in the filter biofilm. If present in sufficient numbers it can out-compete established bacteria populations. Should this happen then there is little option but to sterilise the biofilter with hypochlorite or something similar to remove the pathogen before restocking occurs. Some may argue that this should be done after any serious disease outbreak as part of the control regime.
© Copyright Adrian R. Tappin
Created July, 2005
Updated December, 2008
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