The only way a 6 y/o clown has brook (or velvet) is if it was recently introduced by a new fish, or the encysted stage hitchhiked over on a new rock/coral/invert. There's also the possibility of cross contamination from another tank (ex. wet hands), which can inadvertently introduce the free swimming stage.
Standard treatment for brook is formalin (37% Formaldehyde) treatment. It's also a good, but temporary, treatment for velvet. Most opt to perform 30-45 min formalin baths on the fish (every 2-3 days), as prolonged exposure (i.e. in-tank treatment) can be rather harsh and quickly depletes the tank water of oxygen. You will need to put the fish in a new tank with "clean water" after each bath to prevent reinfection; or treat the QT with something like Chloroquine phosphate to shield the fish from reinfection. Alternative treatments for brook include Acriflavine & metronidazole; I know people who have successfully treated brook with those, but I have no first hand experience.
Velvet can be difficult/tricky to see, which is why I think you are dealing with brook here. Sometimes you will see this gold sheen on the fish, but only if you are looking at the right angle and under the right spectrum of light. Many times if a fish looks to have a really bad case of ich (i.e. completely covered), it is actually velvet. Velvet tends to kill before visible symptoms present themselves, because the dinospores typically invade the gills first and the fish suffocates to death from excess fluid building up. This is why you usually have to rely upon behavioral symptoms (ex. heavy breathing, swimming into the flow of a powerhead, reclusiveness, sensitive to light) for diagnosing velvet.
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