Physical Damage & Wounds
A fairly obvious cause of ill health is in physical damage to the fish, although the causes of physical damage are not always obvious. If a fish becomes physically damaged, leaving open wounds, the area of damage is prone to infection with bacterial and fungal pathogens present in the water. The correct treatment for open wounds can be found in the disease index and the most common causes listed below:
Damage caused by other fishThere are a number of reasons why a fish might attack another fish and whilst situations involving aggressive species and 'bully' fish are indicative of a bigger problem of incompatibility, in many cases seemingly peaceful species can cause damage to each other. Some species, including most shoaling fish, continually have bouts and small fights amongst the more dominant fish in the group. This is part of the fish's natural behaviour and occurs in nature as well as in the aquarium so should not be of any concern, although some damage is inevitable. Whilst in the wild pathogens that affect the damage caused by such bouts may be controllable, in the aquarium some conditions are present that give disease pathogens an advantageous position and may cause disease to take hold.
Bullying and mucus eatingAnother form of damage caused by seemingly compatible fish can occur when a fish shows early signs of illness such as the common immune response of producing excess mucus. In this instance some fish will get a 'taste' for the mucus and continually attack the ill fish, causing further damage and escalating the severity of disease. This behaviour is common with some algae-eating fish and particularly with active natured algae eating loaches.
Damage caused by objectsWhilst fish encounter all types of objects in nature, in the aquarium fish are confined in close proximity to all objects in the aquarium and therefore more likely to come into contact with such objects. Sharp or jagged rocks can cause damage to fish, most often during an attempt by the fish to flee or hide during the normal course of life in the aquarium. To minimise the risk of this happening any sharp objects should be out in the open rather than in enclosed spaces.
Damage caused by incorrect substrateOne of the biggest causes of damage to fish, and often the most overlooked, is an incorrect substrate (gravel). Many fish scavenge amongst substrate for food items and some even bury themselves amongst it. If the substrate is sharp or rough edged, these fish and any bottom dwelling species are prone to constant damage to the underside of the body and the mouthparts. Aquarium substrate should be both smooth and rounded, or a fine-grade sandy substrate in all aquaria.