I think people are confusing healthy as opposed to infection. Apparently we all have cancer within us if so does that make us unhealthy? IMO and FWIW I don't consider the fact a fish may have the odd parasite unhealthy far from it. The fact the fish has a few parasites but in all other respects is in good health swimming eating good colour and normal behaviour means it's able to deal with parasites to the point the fish is not overwhelmed by them. Like Paul B I am sure there are various parasites and diseases lurking in my tank waiting an opportunity to explode but the fact my fish are healthy prevents such an event taking place. I firmly believe the way I keep my fish keeps them in good health. A fish might look healthy displaying some if not all of the above and yet break out in a bad infestation of itch or whatever. Now that is what I would call an unhealthy fish to begin with. If I see a spot or 2 in any of my fish which may happen from time to time with my Royal grammas for instance I just let it run it's course, no need to panic like some and certainly no need for any form of medication. I have been keeping my fish my way for over 30 years now and disease has never been an issue and I am talking never QTd and with about 10 tanks in that time from nanos to 130 gallons.
