It has been said, "take care of the water quality and the fish will take care of themselves."
Truer words were never spoken. There are only two reasons why fish get sick.
2. Bad water quality
Keep in mind that just because the water looks crystal clear, it may still contain deadly amounts of ammonia and or nitrite. In order to avoid problems the ammonia and nitrite must always be 0, zero, none, nadda, no other reading is acceptable. There are other factors in water quality also. pH, Water hardness, softness (GH), carbonate hardness (KH)
The following links will explain in more detail.
How dirty is your pond/aquarium
Think your pond/tank is clean? Take a look at this link!
Click on the link for Using Potassium Permanganate if your pond is really dirty.
Purchase test kits
Here you will find basic supplies plus pertinent water testing kits.