Sorry I know this is an old post, but I'm wondering why you mentioned "hiding from light". I have a YT that has recently (last few days) been hiding under a cave. No new additions of fish. It used to veraciously eat nori and everything, but over the last few days it doesn't seem to care about food, or the nori, and just prefers to swim around back and hide.
I've had it for about 8 or 9 months.. Other fish seem ok and all love to eat. I haven't changed my lighting or anything. Not sure what happened. I did have a BTA that moved around where it used to sleep, and has since moved to the other side of the tank.. is it possible it got stung or ticked by that and is just freaking out? That BTA moved about a week ago.