It is important that you watch your fish regularly so that you are able to spot any changes in their behaviour very quickly. Changes in behaviour can be due to illness.
Regular water quality checks can also help to spot problems before they become serious.
Also bio-security is important – this term covers protection of your fish from possible sources of infection such as introducing new fish into your system, borrowing used equipment, or inadequate cleaning and disinfection following a disease outbreak. All of these factors can be important in protecting your fish.
However, if your fish look as though they might be suffering from a problem, a partial water change is recommended just to increase oxygen and to reduce potential toxins or contaminants.
Always seek advice from your veterinary surgeon, fish health expert or specialist retail outlet.