Health and Disease
If you invest some time in learning about your fishes needs and caring for them properly, you will hopefully encounter disease very rarely. Remember that disease organisms will always be present in the aquarium to some extent, but they will not normally affect a healthy fish.
Diseases take hold when fish are injured or weakened by stress, which may be caused by poor water quality, aggressive tankmates or simply the transportation and handling involved in acquiring new fish. Therefore, the first step in combatting disease is to prevent it in the first place, by providing stable, good quality water, compatible tankmates, etc.
Introducing new fish to an established aquarium is a potential source of new disease organisms, to which the current inhabitants may have no resistance. The use of a quarantine tank is therefore advisable. This allows time to observe the new fish in isolation before adding to the main tank, and putting the rest of the fish stock at risk.
Unfortunately, even when good tank maintenance is practised and a quarantine tank is employed, there may be rare occasions when there is a disease outbreak (like when your young son decides to stress the fish by turning the heater up all the way!). Fortunately, many fish diseases can be cured, particularly if noticed early on. For this reason, it is advisable to observe fish carefully and note any changes in behaviour or appearance. Some diseases have fairly obvious symptoms, and others are unfortunately subtle. There are also general symptoms to look out for - such as a loss of appetite or colours appearing darker or paler than normal. The links below provide details on a number of common diseases, including their symptoms, causes, and possible cures.