I felt like this was a good read... this may be what happened to your Idol...
Some general information on Velvet
It may have been in your tank and when you added the Idol (after having been bounced around from tank to tank for a couple months and been kinda stressed) it gets introduced to your tank that may have had velvet in the tank but the fish are okay with it cause it's not too bad or the fish had it but it wasn't too bad, but the Idol got stressed. It comes in gets the Velvet/loses the battle and you get a huge explosion of dinospores with the added stress of the new tank mate, Velvet outbreak... Just my 2 cents, because you seemed concerned that nothing had changed in a long time but this still happened, I though you might appreciate another view.
From another site "diatomaceous earth filters may also be economically employed to control this parasitic disease" It kills them in the tomont stage. Or a 50 micron filter...