Whilst vets and health experts may have access to laboratories and diagnosis equipment, as fishkeepers in most cases all we have is visual cues to aid diagnoses of problems. A good retailer is an invaluable source of advice to aid diagnoses, but they can only help if you can provide them with the right information. Take a good look at the fish affected and ask the following questions to aid your diagnoses, or to provide your retailer with suitable information;
Which fish are affected?Some fish are prone to specific problems, for instance dwarf gouramies are prone to ulcerations caused by bacterial infections. If this is the fish affected whilst all other fish seem in perfect health then there may be little need to investigate further. Another example might be if several individuals of a particular species of fish, say, neon tetras, are in ill health whilst all other fish appear perfectly well. In this case it may be that the disease pathogen only affects neon's and this will give hints as to the correct choice of treatment.
Have you carried out any recent maintenance?Many diseases are caused by recent events in the aquarium environment. Large water changes, incorrect maintenance, or temperature changes can all disrupt the environment and allow disease to take hold. Thinking back over any maintenance or disruptions may help to hint at the cause of the problem.
What symptoms can be observed?Take a good look at the affected fish and note all the symptoms accurately. Identifying a disease with the description 'it has some white marks on it' is not a good enough description. How big are the marks? Are they round and distinct or fuzzy and irregular? What parts of the body are affected? How quickly did they appear? Do the marks protrude from the body? Is there any redness or accompanying marks? Providing an accurate description of the symptoms is essential to diagnoses and is the difference between ending up with a treatment focused to the pathogen or a treatment that is an ineffective 'hit and miss' product.
Take a pictureIf you have a good retailer and a good digital camera, it may be possible to take a photo (or even a video clip) of the affected fish and ask your retailer to view the photo and provide a diagnosis.