Are you able to quantify / measure the immunity of the fish somehow? How can it be determined that a fish could withstand exposure to certain common parasites like marine velvet or brook?If any of this were true, my and quite a few other tanks would not exist. More parasites means more immunity and immunity is the epitome of fish keeping. Allowing the fish to only die of old age while never exhibiting any disease. What is wrong with that?
To prove it, just look at any old tank. Old is not 5 or 10 years.