How can you tell when a fish is sick?

The key to successfully treating fish diseases is to recognise the earliest signs of diseases in fish and then arrive at a correct diagnosis. If a health problem arises, sick fishes will show a range of changes in their behaviour, feeding activity, and general physical condition. The good news is that no matter which of the >40 000 species of fish that may be seen in practice, the clinical signs presented are very similar. But the bad news is that the clinical signs displayed are often non-specific, and are insufficient to arrive at a diagnosis.

Before clinical signs of illness are discussed, it is important that veterinary practitioners are familiar with the appearance of healthy fish. The following are characteristics displayed by a healthy fish:

When fish are unwell, they may display all or a combination of the following behavioural changes (Figure 1):

Some of these behavioural changes provide the observer with some clues. One of the earliest signs of disease is inappetance. Inappetance may be a result of incorrect environment (e.g. water temperature being too high, or too low), a range of infectious diseases and more. Fish that are piping at the water surface are likely to be suffering from oxygen deprivation or carbon dioxide toxicosis (Loh and Landos, 2011; Loh, 2014). This may be the result of changes in dissolved gases, or if there is compromise to their respiratory system (e.g. parasitic infections of the gills, nitrite toxicosis and acidic water). These affected fish would also present with increased respiratory rate and effort. Fish that flash (scrape their bodies against objects) are reacting to skin irritation which can occur with parasitic infestations (Wildgoose, 2001; Loh and Landos, 2011), such as fish lice (Argulus), skin flukes (Gyrodactylus spp.) and a range of protozoa (e.g. white spot disease). All these same agents also cause fish to hold their fins clamped (Loh and Landos, 2011; Loh and Landos, 2014).