Fish health is not complex; keep stress to a minimum and the fish have the ability to fight off most disease attacks and remain healthy. The key word is stress and this can be present in many different forms, almost all of which are within the control of management. Prior to introducing fish into an environment we also need to ensure that they are free of parasites, reducing the risk that parasitic load itself can become a stress factor.

The water parameters must be excellent and appropriate for the species you are farming, the feed must be high quality and provided in sufficient quantity, and all handling needs to be done gently. Social interaction needs to be appropriate and the theory that lower is better is not always true. Many fish species are communal and feel stressed at low densities. Every species I can think of performs better when surrounded by other fish than it does on its own, even if the companions are of a different species.

When we notice that our fish are unhappy it is therefore imperative to investigate the range of causes and isolate which factor is creating the stress in order to decide upon the most appropriate method of reducing the stress and allowing the fish to recover. All too often fish farmers are tempted to treat the conditions with what they perceive to be the best approach, or perhaps a silver bullet, without first investigating the true cause factor. Most treatments involve water temperatures or chemical additions that are themselves stressful to the fish, and should not be administered without careful consideration. If you use an inappropriate treatment this adds stress to a fish that is already dealing with sub-lethal stress levels, and the additional trauma can be the final straw that kills it.

 
The solution is fortunately simple, keep the fish under optimal conditions and they will almost always be happy and healthy. If you do suspect a problem investigate first to identify the cause of the problem and only then decide on an appropriate path of action to follow.