I'm no ich expert. I've only briefly seen it in my tank and it "went away" after a few days or so.
My understanding is that a tank can have ich, such as mine, and with a healthy system the fish's immune system will suppress the ich.
Sounds like you might have some stressor that caused the flair up? Did you add any coral recently? It can come in on coral.
Did you check the water quality? Any swings in salinity or temp?
