The best thing you can do is become familiar with the signs of illness. Look for such symptoms as unusually rapid breathing, changes in activity level, clamped fins, difficulty swimming or orienting themselves, loss of weight, or change in eating habits. All of these may be signs of disease. The fish owner that is familiar with the normal appearance and activity of their fish can often spot disease in time to treat it.
It's also wise to have access to, or keep your own books on fish health. That way you'll be better prepared to diagnose and treat problems that may occur in your aquarium.
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