I hate to get involved in this and it is nice your tang is 4 years old in an ich free tank.
My hippo in this picture was 10 years old in an un quarantined ich infected tank.
That red fireclown is still alive and him and his mate are 30.
This mandarin was 10 here in the same "infected" tank
This copperband was 10 in the same tank
My new Hippo here is still a baby at 3 years old
Showing a fish a few years old in a quarantined tank is fine and proves a fish can live without parasites for a third of it's lifespan. You are a good aquarist and those fish probably love you. Now if those fish die of old age while never getting sick. That will be a success and prove your point.
So far I have not seen to many or even one quarantined fish that died of old age and was never sick. And spawn if it is they type of fish that can spawn.
PS, I also moved this tank. Fish and all.