Bacteria are microscopic single-celled organisms, which are present virtually everywhere from inside intestinal tracts, aiding in digestion, to the surface of your skin, and the inside of aquarium filters. In the aquarium bacteria are important in the process of breaking down waste matter and dead organic material, releasing nutrients which can be used by higher organisms such as plants and algae, which in turn are eaten by the fish. Bacteria are also responsible for colonising the surfaces inside a filter and processing the waste ammonia and nitrites produced by fish waste. In this respect, bacteria are vital to the well being of the aquarium and the survival of the fish. Water quality is closely related to the bacterial population in an aquarium. In a new aquarium there is a very small population of bacteria so any waste products quickly release toxins that may not be immediately processed. In a matured aquarium, which has been running for several months there is a balanced population of bacteria, both in the substrate and on surfaces within the aquarium which can adapt quickly to changes, processing excess wastes and keeping overall water quality stable.
In other instances, bacteria can be disease-causing pathogens, which under the right conditions can become a serious threat to a fish's health. Disease causing bacteria are always present in the aquarium but are normally kept at low levels or prevented from 'taking hold' by the fishes own immune system. During times of stress, such as after transport, or when water quality is below acceptable levels, a fish's immune system becomes severely diminished. It is at this point when naturally present bacteria can multiply and cause disease. The diseased fish, if unable to fend of the bacterial infection, will become a host for the bacteria to multiply upon, when this happens the other fish have a much increased bacterial population to fend of and the disease can quickly spread to other healthy fishes.
Water conditions are more often than not the root cause of bacterial diseases taking hold but providing water conditions are returned to normal, there are many antibacterial treatments available which provide an effective means of treatment. It is because fish that are under stress are more likely to succumb to bacterial infections, that new fish are the most likely to become diseased. New fish may also carry bacteria that the other fish have not encountered; this is particularly common when fish that inhabit different areas of the world are brought together. This situation means that a perfectly healthy fish has the ability to cause other healthy fish to become diseased, if they have no natural immunity to the new bacteria. Whenever new fish are added to the aquarium it is good practice to keep a close eye on all the aquariums inhabitants, in case of a transferral of bacterial infections.