Stress can be argued as the most common cause of disease problems in the aquarium. When a fish becomes stressed, its ability to fend off disease pathogens is heavily reduced, and it becomes open to infection by pathogens already present at natural levels in the fishes body or in the surrounding environment. Stress can be physical (e.g. damage to the bodies tissues) or emotional (e.g. fear, shock) and is loosely defined, being applicable as an additional cause to any situation of ill health, since ill health itself can be a stressful situation. When a fish is ill, if the underlying cause is stress, treatments will not work until the stress-causing situation is removed.
Noises and vibrationsOften caused by people tapping on glass, doors banging, stereos, televisions and other appliances, noises and vibrations have a great effect on a fishes sensitive nervous system.
Incorrect decor in the aquariumDepending on the type of fish you keep, they will have a preferred physical environment such as a rocky tank, planted tank, open spaces, enclosed spaces, fast flow, slow flow, shaded areas, soft substrate, hiding spots and so on. Not all of these are compatible and fish that require quiet heavily vegetated tanks may become very stressed in tanks with large open areas and few hiding spots.
Incorrect tankmatesBoisterous and aggressive tankmates will cause severe stress to fish in the aquarium since they cannot escape and will feel constantly under threat. Situations like this often escalate since the attacking fish will continually attempt to drive other fish away, not knowing that they cannot leave the confines of the aquarium. The bullied fish has no option to retreat and hide, causing stress and reducing the fish's ability to feed. Some fish like to live very quiet, slow moving lives, and these species will become very stressed by larger, more active fishes, even when no aggression is shown by the larger or more active fish. Another stress causing situation occurs when shoaling fish are kept in small numbers instead of large groups
Incorrect water conditionsFish naturally all live within a set of water conditions found in their natural environment which include temperature, pH, and Hardness. Whilst many fish can adjust to a different set of conditions quite happily, some cannot and if kept in the wrong set of conditions, will become stressed, weak, and prone to ill health.