What are ulcers?
An ulcer is defined as a break in the skin extending through all the layers, which fails to heal and is often accompanied by inflammation. The typical skin ulcer has a central area of tissue erosion that may be shallow or deep – in many cases exposing the underlying muscle. Around the edge of the lesion is usually an area of whitish necrotic (dead) tissue and debris, with a large outer zone of inflammation extending into healthy tissue. The scales surrounding the lesion may be raised or infected. Without treatment it gets progressively bigger. In most, but not all, cases they are caused by opportunistic bacteria already in the pond or tank.
Treat all the affected fish
It is often the case that there are many more infected fish than initially thought. It is important that all of those affected are treated at the same time otherwise the untreated fish will in turn get worse as well as acting as a reservoir of infection. This is often the situation where an outbreak of bacterial disease goes on for months, despite continually treating the most obviously affected individuals. To the owner it looks like fresh outbreaks despite his best efforts, when it is really just an ongoing chronic infection.
Aim for a 90% survival rate or higher
I should stress the importance of tackling bacterial diseases such as ulcers in a systematic, thorough way. Simply applying dartboard medication, in which a range of different treatments is tried in succession, is unlikely to work and in all probability will simply make matters worse. When dealing with bacterial ulcers we should be looking for at least a 90% survival rate. If the survival rate is significantly less than this it is most likely due to inappropriate treatment rather than bacterial infection.
HTH