Tangs - always prone to Ich or can it be erradicated?

[wooden_reefer]

So how do you keep ich out of your display in the first place, since this fish apparently took months to show visible signs?

That sometimes it takes months (generally two or fewer) for significant infestation to show up does not mean that it must take equally long to eradicate ich. If ich is not checked by the aquarist, there is always a fair element of chance.

You should remove the element of chance during active treatment to break the cycle of ich to the greater extent, even if not actual zero to virtual zero.